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Discussion Starter · #1 · (Edited)
Here are some of my thoughts on algae's appearance in our tanks and its possible relation to nutrient deficiencies.

Observations:
There are several interesting scenarios that we are all familiar with:

1) The healthy tank, uses chemical fertilizers according to EI or PPS-Pro with very little algae in the tank.
2) Established soil tanks, very little algae.
3) Tanks with plants that don't grow well and lots of algae.

Conventional thinking used to be that it was an excess of nutrients that caused algae to appear. Lots of effort was put into eliminating excess nutrients. Frequent water changes, using carbon in our filters, phosphate scrubbers, reduced lighting, etc all reduced these "excesses" and eventually the algae disappeared as well - likely eventually succumbing to resource deficiencies. This particular method of keeping an aquarium works well with fish-only tanks, but does not work well with planted tanks. Plants develop deficiencies and die as well as the algae.

Later on the EI method allowed people to grow lots of plants quickly and over time people realized that an excess of nutrients was not the sole cause of algae in our tanks. This has been shown countless times where people often have 20+ ppm of nitrate, and 2+ ppm of phosphate along with excesses of all the other nutrients in the water column and there are no algae blooms. Under these conditions algae does not flourish so long as there are enough plants in the tank.

Diana Walstad's book - Ecology of the Planted Aquarium mentioned how many species of aquatic plants produce chemical compounds that can inhibit other species of plants. This concept made me wonder if the same could be true of plants vs. algae. After reading through obscure plant and algae journals I found out that this is in fact true as well. Plants do make allopathic chemicals that significantly reduce algae's growth. Furthermore, algae does not seem to be able to adapt to these compounds and so they remain effective even though many generations of algae are made over time.

From my botany class days where we grew mono cultures of various algae species for experiments the standard procedure was to grow algae in small glass containers which contained all the mineral nutrients that we dose in our tanks. The algae we grew developed quickly and grew healthily in these conditions. The major difference between our tanks and those test tubes is - there were no plants in the test tubes. This implies that the presences of plants influences algae's ability to grow and further supports the journal articles mentioned above.

Reference:
Miquel Lürling, Gerben van Geest, Marten Scheffer. Importance of Nutrient Competition and Allelopathic Effects in Suppression of the Green Alga Scenedesmus obliquus by the Macrophytes Chara, Elodea and Myriophyllum: Hydrobiologia Journal, February 2006, Volume 556, Issue 1, pp 209-220
http://link.springer.com/article/10.1007/s10750-005-1168-3

My own research into nutrient deficiencies has uncovered many interesting relationships between nutrients and plants, along with some connections between nutrients and algae. For example, it is possible to give algae deficiencies when a tank is deprived of a nutrient.

During a discussion on what causes BBA to grow in our tanks (a lack of CO2 or an abundance of dissolved organic material in our tanks) I found several interesting articles which described how many species of algae can actually directly consume dissolved organic molecules in place of nutrients like nitrates and phosphates. In other words, algae can feed off organic molecules that plants release and dissolved partially broken down organic molecules.

Discussion:

Putting all of these seemingly unrelated facts together I think many, if not all of the algae outbreaks we observe in our tanks might be explained.

Plants suppress algae growth in a number of ways. They compete with algae by shading them out, by releasing allopathic chemicals into the water column that inhibit or kill algal species and also by growing quickly and not leaking organic molecules which algae can feed on.
Reference:

Brij Gopal, Usha Goel. Competition and allelopathy in aquatic plant communities. The Botanical Review Journal: July-September 1993, Volume 59, Issue 3, pp 155-210
http://link.springer.com/article/10.1007/BF02856599?no-access=true

This is likely the reason why EI and soil tanks work so well to suppress algae. The EI method prevents plants from ever running out of any nutrient. This encourages fast growth which allows plants to shade out algae, to have plenty of excess resources to make the costly allopathic chemicals which stunt algae and also allows plants to maintain healthy intact cells which do not leak out their organic contents into the water column. These processes all combine and inhibit most algae species in our tanks from growing and taking over. Algae has to struggle to make a living and is generally confined to tiny patches near the substrate which receive enough light and which do not receive a lot of water flow (and the allopathic chemicals that are constantly being supplied in the current).

Soil tanks work in a slightly different way. After soil tanks have finished their break-in period where nutrients leach out of the soil and accumulate in the water column they become mature tanks which are extremely stable over the long term and generally have very few algae problems for many years (until the soil nutrients become depleted). Soil tanks feed the plants (via the roots) and ensure they never become deficient which in turn allows plants to shade out algae, produce expensive allopathic chemicals and maintain healthy cell walls that don't leak. However, soil tanks also tend to maintain a water column that is constantly devoid of nutrients. This nutrient deficient water column adds a fourth safeguard against algae by forcing algae to try make a living in nutrient deficient conditions on top of the other three inhibitions. Since algae does not have the ability to grow root systems it cannot move nutrients out of the soil layer through the substrate cap and into the water column area where the light that drives photosynthesis is located. These four factors combine to create an incredibly stable long term tank system that has very few algal issues. This method also has the added benefit that fertilizer delivery is automatic and does not depend on the aquarist correctly measuring out enough chemicals or remembering to fertilize their tank. This is also likely the same reason that Amano's ADA Aquasoil works so well, producing beautiful planted tanks with no algae for many years.

This idea explains why algae seems to pop up when tanks run out of nutrients. For example, when the CO2 runs out in a moderately planted tank and you see algae develop. This is not because high CO2 levels directly harm algae (algae uses CO2 as a nutrient just like plants do), but rather because the plants cannot spend as many resources on producing allopathic chemicals which normally inhibit the algae. On the other hand, if you have an extremely densely planted tank and you lose CO2 you might not see any algae - because there are enough plants to maintain a fairly high level of allopathic chemicals even though each plant is making less than it did before when CO2 was plentiful.

When a mineral nutrient runs out in a tank, like nitrogen (KNO3) for example, plants and algae begin to develop deficiencies at a fairly similar rate. Plant's develop the normal symptoms of nitrogen deficiency (decaying older leaves and smaller new growth which eventually stops growing). However, as the plant's old leaves die they release all of their proteins (high in nitrogen) and other contents into the water column where algae can directly consume it and use it for growth. This relieves the nutrient deficiency condition for the algae only. Unfortunately plants are specialists and cannot consume organic matter directly. They require nutrients to be present in mineral form (KNO3, etc) and so they remain nutrient deficient and continue to release more and more organics into the water where algae uses it. Interestingly this also seems to correspond to the location where certain species of algae will grow. Most notably BBA. BBA seems to grow on older parts of plants, at the nodes between stems where cellular connections are weaker, on filter return pipes, and on slow growing species that maintain their leaves for long periods of time. These locations are all locations where the plant cells are old, weakened and most likely leaking organics into the water column. The filter return line is loaded with organic material from the filter where bacteria have partially digested fish food and dead plant matter. In addition to the organic nutrient source plants can no longer spend resources on producing allopathic chemicals or continue to grow and shade out algae. The collapse of normal algae inhibition enables algae species that used to be suppressed by multiple processes to suddenly take off, flourish and ruin our tanks.

This idea might also explain why algae blooms are notoriously difficult to predict. After all, it is difficult to monitor our nutrient levels accurately and continually. So one line of reason suggests that the cause of "random" algae outbreaks might in fact be caused by temporary nutrient deficiencies. If plants in our tank run out of nutrients in the water column temporarily, they switch over to using up their reserve nutrient stores and start cutting corners when growing, reducing their growth rate, reducing allopathic chemical production (a luxury at this point) and preparing for starvation. This reduction of allopathic chemicals might temporarily allow opportunistic and fast growing algae species to bloom. This may also explain why we generally tend to see only a handful of algae species in our tanks compared to the thousands of species of freshwater algae that exist. The algae species we commonly get are the best species at taking advantage of sudden lapses of inhibition, blooming quickly and then going dormant until favorable conditions return.

Conclusion
Ultimately the take home message here is that if we have enough plants in our tanks, keep our plants well supplied with nutrients and never let them run out then algal issues will be minimized or completely eliminated.

Here is an image of what I just said above for all you visual people:
 

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Very thoughtful--a unified theory of algae growth in planted aquaria. It is especially refreshing that you do not have a "sectarian" view, and explain how different methods can produce the same algae-free result in somewhat different ways. If this thread does not take off, I will keep bumping it to the top.
 

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Discussion Starter · #4 ·
Thanks. I'd love to hear more discussion on this idea and see if it makes sense as it did to me when I wrote it. I think it does a decent job of covering all circumstances of when algae appears, but perhaps I've overlooked something. I can dig up the algae papers that I mentioned above and add them to the original post for completeness if anyone wants to read them and see for themselves. Might take me a while though since I'm studying quite a lot these days and only have an hour a day off.

I'm waiting for niko to log on and read this. I feel like this topic is right up his alley because it also relates to water flow and why algae tends to build up in areas that have bad water flow (less allelopathic chemicals reach that area).
 

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By these theories, bad flow will support the algae in not 1, but three ways:
  1. Less allopathic activity: As you already said
  2. Nutrient depletion: Plants will depend on diffusion instead of flow, less efficient.
  3. Increased Organics: Mulm will settle in the area. As it breaks down, it won't be diluted into the rest of the water and leaves a higher concentration of DOC for algae

I suppose bad flow can also cause decreased total PAR since there won't be a swaying action to allow light to temporarily penetrate any canopies. Instead of shifting shadows, you get stagnate shadows that keep an area permanently dimmer.

EDIT: BTW, I would love the link if you get the chance. No rush.
 

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Discussion Starter · #6 ·
Wow, good points. I hadn't thought about the other two factors in low flow areas, but that makes a lot of sense since mum accumulation wouldn't help plants as they can't consume organics directly.

Another interesting point is that we seem to get the same types of algae in our tanks despite the fact that there are thousands of species out there that we are undoubtedly exposed to. Why so few? I'd bet the species we get are probably capable of eating organics or are adapted in other ways to our tanks that give them more tolerance to these inhibitions than other algae species. What do you think?

At aome point I will draw a diagram of this theory, I think it will help explain it better. I'll try work on that and post it later.
 

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http://link.springer.com/article/10.1007/BF02856599

Hope that's the right article, you'll need access to the journal to read it though. Basically points out a lot of what you're saying about the multi-pronged algae suppression. I don't think it went into as much detail with organics, etc. Very interesting points you've made there...

I like that you bring allelopathy into the equation, I feel it was unfairly pushed out of our attention by people who didn't really understand what it was or how it worked and everyone just sort of followed and forgot about it. So here's an article you might like as well http://link.springer.com/article/10.1007/s10750-005-1168-3

Same paywall deal though, sorry.

So here's a thought... Perhaps plants which produce greater amounts of allelopathic chemicals against algae could be grown out in a sump tank?

Anyways, I think your post should be bumped until everyone in the hobby has read it. Perhaps even stickied!
 

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Discussion Starter · #8 ·
http://link.springer.com/article/10.1007/BF02856599

Hope that's the right article, you'll need access to the journal to read it though. Basically points out a lot of what you're saying about the multi-pronged algae suppression. I don't think it went into as much detail with organics, etc. Very interesting points you've made there...
Wow fantastic articles Skizhx. I just downloaded them both and will try read them as soon as I can, though the first one is quite long (57 pg). I had not read them before but they definitely seem to support parts of my idea quite well, especially the second article you posted (Importance of Nutrient Competition and Allelopathic Effects in Suppression of the Green Alga Scenedesmus obliquus by the Macrophytes Chara, Elodea and Myriophyllum). I still need to dig up my other references to add them all to the main article with yours. I really appreciate you looking into the articles

I like that you bring allelopathy into the equation, I feel it was unfairly pushed out of our attention by people who didn't really understand what it was or how it worked and everyone just sort of followed and forgot about it. So here's an article you might like as well http://link.springer.com/article/10.1007/s10750-005-1168-3
I think allelopathy certainly has more of an effect than we realize in our tanks. I used to grow algae cultures in the lab using the exact same nutrients as we use in our tanks and the algae grew very well and quickly, however, algae doesn't usually grow at all in heavily planted tanks that are well fed. The fact that algae seems to struggle when grown with healthy plants is one of the only well supported conclusions we have in the hobby with regards to algae. Your second article also strongly supports this conclusion.

I don't think it is possible for hobbyists to test for specific allelopathic chemicals since we lack the funding and machinery, however, the research that has already been done seems to strongly support the idea that it plays a huge role on algae control.

Along with allelopathy I also believe that certain species ability to directly consume organic material as a nutrient source might be an important concept to controlling algae issues in our tank. JeffyFunk's Organics Analysis thread about BBA and dissolved organics thread (http://www.aquaticplantcentral.com/forumapc/algae/89126-organics-analysis.html) caused me to do some research into algae and its ability to consume organics directly without having bacteria fully digest them to mineral salts. The studies I read confirmed that many species of algae can consume organics as nutrients, which makes sense since when plants start to die for whatever reason (nutrient deficiencies/damage/etc) they release organic molecules that algae presumably can feed on. Which is why bad plant growth (or to a lesser degree a fish's death) tends to be linked with algae blooms.

So here's a thought... Perhaps plants which produce greater amounts of allelopathic chemicals against algae could be grown out in a sump tank?
I think this could definitely help. Presumably the more plants the more allelopathic chemicals the less algae.

I personally believe that high tech soil based tanks are probably the most stable method of setting up a tank since they have 4 controls against algae vs. water column fertilization methods which have only 3 (see the diagram above).

Anyways, I think your post should be bumped until everyone in the hobby has read it. Perhaps even stickied!
I hope others find it useful. I'm simply happy that people are reading it and talking about the possibilities (not to mention citing good studies for evidence!). Perhaps this thread can eventually be used as a framework for how to deal with algae. I suppose time will tell.
 

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The focus of the orginal post is on plants, algae, and nutrients. But the aquarium has more "moving parts".

In the last decade these "parts" have been ignored to the extreme. A few examples are filters "rated" by brand name, water movement setup in all kinds of inefficient ways, the cleaning crew being viewed as a few Amano shrimp + a few Otos, the "high" or "low" PAR light readings are based on tanks run as an incomplete system on the brink of developing issues, and so on.

Zapins talks about nutrient levels that are sufficient to sustain good plant growth. For a decade now these "sufficient" levels have been along the lines of the following concentrations: N= 5-20, P=0.5-2, Fe=0.1-0.5. These suggested concentrations can be seen even on the Fertilator screen as a general guidelines. But there are at least two problems with these values:

1. Plants only care if nutrients are available (not how much is available).

2. Any factor that disrupts the good plant growth leads to explosive algae growth, often an overnight occurrence.

More about point 1: The water column fertilization of the ADA approach is to provide the bare minimum of fertilizers. It is about nutrient availability but not excess. If plants indeed required more than the minimum we would not see ADA tanks as lush and healthy as they are. We will not see Crypts and other plants completely covered with Oxygen bubbles when the water flow is turned off. There will be no videos of ADA tanks with strong water flow (making the fish struggle against it) that also show heavy pearling at the same time.

More about point 2: In a tank which provides the plants with all needed nutrients in a bare minimum amounts algae outbreaks are rare. If they happen the "fix" is both manageable and predictable. The algea does not explode overnight. One to three water changes reset the water column to where all algal growth stops. Plant growth can stop too but later than algal growth - the problem is completely manageable.

The important thing here is to be open minded to the reality that nutrients are not everything. If they were then resetting the water column to where algae and plants stop growing will always result in another algae outbreak as soon as nutrients are added to the tank. Both plants and algae would grow. But algae will not grow if certain compensation factors are in place.

The explanation of why plants can grow and algae can be absent is not clear. It is surprising that after at least a decade of bright and educated minds being involved in this hobby we still do not understand how it all works. Allelopathy is a very reasonable idea. But it seems to work only in tanks with lots of plants. It could also be reasonable to wonder why allelopathic chemicals are not available in bottled form - fake or not. Maybe the thought direction should be more holistic than nutrient concentrations, organics, and water changes.

A more practical view on how plants can be grown in absence of algae is the one in which the aquarium is seen as a system of interacting parts. All of them need to be setup to work in a certain way. In this model there is a lot of room for one "part" compensating for another. To clarify - at this point of the hobby's evolution it is clear that too much light does not cause algae, a lush tank is possible without CO2 injection, an overcrowded tank can also be completely algae and maintenance free, and algae growth can be halted by nutrient limitation. Such seemingly inexplicable observations are a result of compensation. But compensation can only take place if all the parts are in place. An incomplete system (for example nutrients and water changes) forces a need for external intervention (mandatory regular maintenance). In that sense a low-light, soil-based, non-CO2 tank is more evolved and intricate than a high CO2, high light, nutrient-ladden tank with little to no compensation potential. Both can stay clean but the stability, maintenance requirements, and algae risk levels are completely different. It would be hard to argue with the fact that only the soil-based tank functions as a system.

I believe that the readers of this thread have been interested and involved enough in this topic to where they can easily make a list of the "parts" that make a planted aquarium work as a system. If one spends the time to make that list it will become obvious how much of the planted tank's setup and internal functionality has been almost completely neglected in the last decade. As an example of the opposite I can account for the success of the tank of a friend of mine that had never had an aquarium before. He bought the system, set it up, and has never had issues for 2 years now. The tank is a salt water reef - a side of the aquarium hobby has worked out all the little details that make aquariums work as systems.
 

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I should get into the habit of bookmarking articles so I can dig them up more often, but awhile back I found an article that was published in the 60's-70's I think, which investigated soil substrates for growing aquatic plants to reduce algae (apparently soil was popular with nurseries back then). It cited two papers which both investigated which nutrients had to be present in the water column. They basically found that with the exception of I think it was potassium, plants don't really need much of anything in the water column if they can get it from the substrate.

With that said, I find the hobby's concept of nutrient competition to be... Basically not how competition works... We strive to ensure that nutrients aren't a limiting resource so the plants can thrive so they'll outcompete the algae for the resource we're ensuring will never be a limiting resource... It's as if we think the plants are getting strong enough to steal the algae's lunch money so they don't get to eat anything. You can't have population suppression by resource competition if the population is quite literally surrounded by the resource at any given time.

Anyways, neat paper I just found about bacteria and algae... http://link.springer.com/article/10.1007/s10750-005-1168-3

Also find it interesting that one of those articles I provided earlier mentions bacterial metabolization of allelopathic chemicals as a factor that determines how much of an affect allelopathy will have on algae.

Not sure why we don't have allelopathy in a bottle. There's a lot of good products that should exist but don't. I really don't think there's much question that it's contributing something to the stability of our tanks though. I think to argue that it doesn't you'd basically have to argue against at least a good 20-30 years of scientific observations, or present a reasonable case for why the plants in our tank aren't doing what they've been found doing in nature. But yes, it would be mostly effective in tanks that contained many plants. Or perhaps plants are capable of producing large amounts of these compounds and releasing them, but the compounds themselves also inhibit their own production (negative feedback). In other words, the plants would be cooperating as a community to produce enough of these chemicals to protect the community from algae invasions, while also minimizing the amount of energy each individual has to expend to reach necessary concentrations. This might explain why our tanks seem to have a better chance of success when we start the tank with a large number of plants from the beginning, but would also explain why tanks with fewer plants still seem to work.

I guess the real question is... Can we run a fish-only tank as if it was planted, and still get by without algae?

I should really be working on something else right now. Oh well, procrastination...
 

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Zapins rightfully pointed out to me that allelopathy is not the only way plants can counteract algae growth.

So it is:

1. shading of algae by plants,
2. allelopathy,
3. fast plant growth allowing the plants to outgrow the algae to at least some extent,
4. healthy leaf structures that do not leak plant juices into the water,
5. water that has been stripped by nutrients by the plants absorbing them.

Shading: By now most of us know well how wildly PAR values vary in different areas of the tank. A tank may appear evenly lit but a PAR meter tells a different story. Shading of algae by plants is not something to be written off easily because quite a few areas in the tank actually have very low PAR. One area that has unexpectedly high PAR is the front glass - usually it is free of plants and the light is reflected by the glass. I think we will all agree that it is quite often that algae appears in the very front.

Allelopathy: It is valid for both plants vs. other plants and plants vs. algae. A tank 80%+ full of plants usually does not develop algae. Allelopathy is never discussed well on any forum but it is hard to write off the 80%+ observations.

Fast plant growth: Here one needs to apply common sense. Plant can never grow faster than algae. But the opposite is true if the algae are kept at bay through a good cleaning crew, organic removal resins, and lean water column. Then plants indeed can outgrow algae. Small dot-llke algae of different kinds can still play dirty though - they can slowly grow on well visible plant leaves and although not a lot of algae mass is present the sight is not the best.

Healthy leaf structures: This one a serious miss in all discussions about plants vs. algae because the plants can look healthy and even grow very fast but still ooze juices into the water. The usual situation is a clean tank with healthy plants some of which have BBA growing on the edge of the leaves. This bring forward the question "What exactly is a healthy plant growth". It seems reasonable to suppose that fast growth does not necessarily mean healthy in every aspect. The discusion should be about "optimal" vs. "fast" growth.

Lean water column: Algae does not grow in nutrient poor water but plants do not grow either. The fine point here is to be able to look at the tank as a dynamic system. This is not about nutrients being absent at all times. This is about nutrients being available to the plants for a short time and the plants being ready to absorb them fast. Most people understand that this is exactly what ADA does despite more than a decade of publishing the data of each one of their aquascapes as being nutrient deficient. Another serious planted tank company - Dennerle - does the same thing - they believe in constant and very fine addition of fertilizers to the water with the idea of a constant consumption of the nutrients to where the water appears nutrient-poor.

At the bottom of the second diagram in the original post here is a red circle with crossed lines. It describes a situation in which the algae gets ahead of the plants despite lack of nutrients. This happens when the water is void of nutrients and the plants start to deteriorate because their organic compounds leek into the water. This is basically a slow death (the cell walls become permeable). But the organics released by unhealthy plant leaves may not cause algae either. In the last six weeks I ran a tank by making sure that both inorganic and organic compounds are at minimum (RO/DI barely reconstituted with K/Mg/Ca and very efficient activated carbon). What I saw was slow creep of encrusting algae on the plants, no algae on the glass where sun hit the tank, and eventually deterioration of the plant leaves. But the algae did not take over the tank in a few days. Basically all that was needed to get the plants going was a minor adjustment of the nutrients to where the plants grow and replacing the activated carbon with organics-removal resin. It is pretty clear that to do the seemingly impossible - grow plants and no algae one needs to make sure that the tank is very clean, well lit, and supplied with nutrients only as needed. Yes, you could claim that the plants must have all that they need but by no means in excess at all times because that both does not allow for easy problem solving should algae appear and because plants, even demanding ones, grow healthy when conditioned to regular minor additions of nutrients to the water. The key fine point here is "conditioned" - you can find enough useless information about how plants reorganize their internal structures when an element is abundant in the water or not and how the light intensity plays a role in this restructuring too. Information like that has been available for ever but never really seen in good light. So it would be reasonable to try to run the tank as both holistic and fine-tuned system.
 

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Hi everyone,

I just want to express my gratitude for such a thoughtful and important thread. You guys really know your stuff!

This is cutting edge material.

I am not a biologist but my background is firmly rooted (sorry about the pun!) in science. I like to understand what is going on in my tank. Algae😉growth in my planted tanks has always been an enigma but slowly I am managing to keep things under control.

Now, why is it that algae never grows on my substrate (JBL Manado) but a small amount does grow on the rocks, which are chunks of natural red jasper?

Thanks again.

Y.
 

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How do diatoms react to same these variables? I picked up some from some plants in a slow moving section of a river (I thought it was mulm/silt covering the plants) Now my tank is covered and brown spotty goo That I have been scraping off the plants and rinsing out of clogged filter floss every day until I get some ottos to eat it. I have 0 ppm nitrogen of all kinds, floating plants start to show yellowing of old leaves quickly (I only have snails so no other sources of nutrients other than the mts), so I doubt there is much of any nutrients in the water column. Everything else is health and growing, but the diatoms don't seems to care about light or extra nutrients, except maybe silica which is in every inert substrate or cap. I also have little to no other algae, a tiny bit of fuzzy fungusy stuff here and there the snails love, and I had 1 clump of what I believe was staghorn algae that I pulled off a plant and let the filter suck up. Tank has been cycled for several weeks.

Has anyone found a way to prevent diatom problems? I wonder how they stay under control in nature other than a few species of fish or inverts that might eat them in various areas of the world, or do they just end up being washed into the ocean?
 

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Now, why is it that algae never grows on my substrate (JBL Manado) but a small amount does grow on the rocks, which are chunks of natural red jasper?
Hi Folks,

I am still trying to answer my own question above.

In order for algae to grow on an inert piece of rock, there must be available nutrients in the water column. And yet, there is no algae on the substrate or even a piece of bogwood in my planted tank. Why?

When algae grows on aquatic plants, it makes me think of orchids growing on trees - epiphytes. In fact, Wikipedia has this to say:

"However, there are many aquatic species of algae, including seaweeds, that are epiphytes on other aquatic plants (seaweeds or aquatic angiosperms)".

Questions, questions...

Y.
 

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I have seen algae grow on the crushed lime you add to cement, pave roads with, etc when nothing else grows in it. I don't know why, but some kind grew all over a bunch in a jar I tested that was filled with dirt that was composed mostly of the dust from the gravel along with the said gravel as a cap and the algae only grew on the grey white gravel after several weeks of sitting in a sunny window.
 
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