How old is your tank? After a year or so, you don't have to worry about the substrate having too many nutrients. That's when a lot of hobbyists start worrying that it doesn't have enough nutrients.

 

It's only 5 weeks old. As I stated, conditions right now are great with no serious problems. I'm looking at what the future might hold.

Your answer confuses me. The decomposition of tank waste will continuously add to nutrient levels of the substrate. If the Bacopas are only pulling small amounts of nutrients out, it seems that the substrate would eventually "fill up" with them. Or are these substrate nutrients dissipated in some fashion other than by plant uptake?

 

I think we're parsing definitions. "Tank waste" includes every bit of organic matter that goes into your tank. Thanks to fish poop and spoiled food, your tank will never be at a loss for things that will decompose. Over time, however, less and less of that will be coming from the potting soil you rinsed and flushed in order to make it compatible with raising fish and other livestock. I agree with @Michael that it never hurts to add more plants.

 

This probably isn't anything to worry about, but if you are concerned add some plants that have strong root systems. There are many possible species, but some of my easy favorites are Vallisneria, Cryptocoryne, and Echinodorus.

 

After doing further research ( I hate scientific documents!) I've decided that it IS something that should be taken into account. If the nutrients aren't removed, they will 'overflow' the substrate, causing sudden unexplained algae blooms and anaerobic 'burps'. The stuff has to go somewhere.

I've ordered a couple of pots of Dwarf Sagittaria to start with.

 

Big question: Does your substrate contain soil? I’ve read you’re using “pond soil”. The reason why it’s important is “gravel only“ tanks create a runaway nutrient build up and toxicity. Soil-based substrates don’t. Why? As johnwesley0 said, ”beneficial bacteria”. Some aquarium substrates like Fluval contain NO soil. Soil-free substrates don’t sustain plants and beneficial bacteria like soil substrates. The soil-free gravel substrates eventually accumulate fish mulm, it becomes toxic and rots creating a haven for anaerobic bacteria. This kills plant roots and leeches toxins and excess nutrients into the water column. After a few years you need a tear down.
True Walstad tanks are “dirted”; they contain soil.
To answer your question directly: ”How do you prevent nutrient runaway in a (dirted) Walstad tank?“ We plant fast growing, low-tech plants (like crypts and swords) that need semi-frequent pruning. By removing excess plant material (biomass) you’re removing those nutrients with it. This is also why floating surface plants are key, (like frogbit, red root floaters and water lettuce, etc.) The constant removal of growing plant material in addition to beneficial soil bacteria, keeps the nutrients in check. Simply put, growing plants consume nitrogen.
The question you have to answer is, is your bagged pond soil suitable for a Walstad-style aquarium build?

 

I didn't use potting soil, I used pond soil straight from the bag. Please tell me what exactly is lessening the nutrients? A handful of stem plants pulls very little from the substrate. All of my other plants pull exactly zero nutrients from the substrate.

Beneficial bacteria.

 

Those are creating the nutrients. Just like in the water column.

 

This is baked clay “Pond soil”?

 

I used Aquascape's Pond Plant Potting Media. It came highly recommended.

 

Ok, I finally found it, knew it was in there somewhere...
From "Ecology of the Planted Aquarium"
Chapter 2 - N Source for Best Growth
Section 3 - Ecology and N Source Preferences
"In the aquatic environment, however, sediments are anaerobic and NH4+ predominates. Ammonium, not nitrates, tends to accumulate, because anaerobic conditions discourage nitrification and encourage denitrification. Since NH4+ naturally predominates in the aquatic environment, most aquatic plant species have developed an ammonium-based nutrition."

This states the Ammonium, being produced by the anaerobic bacterium, will accumulate. It also states the sediments are anaerobic, which would preclude the aerobic bacterium in the water column from acting on them. It then states that plants will feed on the ammonium.

It is widely recognized that stem plants draw very few nutrients (Ammonium) from the substrate.

Therefore, I reason that only heavy root feeders can reduce the nutrients in the substrate, and care should be taken to make sure there are enough of them to handle the load. Just like you do with plants that draw from the water column.

 

Those are creating the nutrients. Just like in the water column.

They are creating less toxic nutrients. This was the standard operating procedure for most gravel-based aquariums before Diana Walstad came along and demonstrated by useful experimentation and documentation that you could save yourself a lot of water changes by combining soil-based substrates with heavily planted tanks (thus, the name of her book.) The point is to get off to a good start by planting a lot of fast-growing plants, if you want to cut down - or even eliminate - the amount of a particular nutrient called, nitrate being produced.

Therefore, I reason that only heavy root feeders can reduce the nutrients in the substrate, and care should be taken to make sure there are enough of them to handle the load. Just like you do with plants that draw from the water column

You seem reluctant to take "Yes" for an answer here. Hope we've been of some help.

 

I have only heard "It doesn't matter, do it if you want to".
There are no nitrates being produced by the substrate.
So far I've received zero help from this forum.

It's the weirdest thing, seems like people want to put deep substrates in their tanks and then ignore how they work. It's not just this forum, I don't mean to pick on you guys.

There are many, many stories on the internet about Walstad tanks suddenly developing serious problems after a year or two. It usually results in the tear down of the tank. Should we not, as a community, try to address this? The answers are in Walstad's book.

 

Lawnboy, you are being argumentative. There are many stories on the internet, so I will tell one. I have Walstad tanks that have been set up and running continuously for 15+ years. Plant and fish species have changed over the years but the tanks have never been torn down. I keep a mix of stem plants, floating plants, and deeply rooted plants and have never had problems with anaerobic substrate or excess nutrients in the substrate or water column. At times I have done stupid things, but the tanks always recovered.

The very first Walstad tank I set up did fail and had to be torn down. But I made really serious mistakes: substrate too deep and lots of drift wood, some buried deeply in the substrate. I wanted a particular design which violated the basic Walstad method.

I do push the method a little. I keep too many fish, and I have lots of circulation and some mechanical filtration. But this has only caused problems during power failures when oxygen levels dropped too much. In the absence of light and circulation all the organisms in the tank continue to respire. I lost some fish that way, but when the power came back on the tanks resumed their normal stable condition.

 

It's the weirdest thing, seems like people want to put deep substrates in their tanks and then ignore how they work. It's not just this forum, I don't mean to pick on you guys.

Walstad substrates shouldn’t be deep. 1 inch of soil capped with 1 inch of gravel. If someone is using more than that, they aren’t following the “plan”.

There are many, many stories on the internet about Walstad tanks suddenly developing serious problems after a year or two. It usually results in the tear down of the tank. Should we not, as a community, try to address this? The answers are in Walstad's book.

I’m willing to bet those many stories aren’t from this forum and those individuals are taking liberties with the Walstad method. Generally those people are set in their ways with high tech setups and are ignoring key factors for a healthy Walstad-style tank (such as using soil-free substrates from the start). Many people with appropriate setups have problems keeping nutrients in their tanks. Solutions include extra fish food, root tabs and other fertilizers. Your tank sounds like it’s on the right path. Don’t be surprised as your plants grow and mature that you’ll have 0,0,0 for ammonia, nitrates and nitrites.

 

The best way, in my experience, is to deal with problems as they occur

It's very hard addressing a hypothetical situation.

 

It’s also easier to just let good things happen 😂