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· Registered
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Discussion Starter · #1 ·
Today I was asked why a planted tank requires the water to be changed so often. I mumbled something about organics and removing molecules that somehow supress the plant growth.

Planted tanks that do well despite the rare water changes puzzle me. What it is in those tanks that makes them run clean and with great looking plants? In the last few months I've been neglecting one of my tanks to the point where the water gets yellow from the evaporation and the lack of water changes. The tank is sparkling clean.

So, why exactly do we have to change the water in a planted tank? And I mean a tank that consumes all the nutrients in a healthy manner.


· Registered
3,252 Posts
The first reason is to replenish nutrients and minerals that have been depleted. If we consider a low ligh, non co2 tank then the water change removes excess built up nutrients and organics that would lead to algae. Things like NO3 and organic carbons that would be benifical to algae an dpossible cause problems with fish. in the low light tank, this as assuming a balanced tank, then fish waste/ food is broken down by the biological colony in the tank and the broducts of this is used in equal amounts by the plants. These are P, N forms, and those organics you mentioned:)

A balanced low light tank often does not get water changes for months. The main reason to change the water here is to reset the hardnesses. Plants use the Ca, Mg, and many other traces they need are stripped directly from the water's "?hardnesses" Some plants, in low CO2 situations also strip a Carbon source from the kH part of the water(CO3)

High light tanks are a diferent kettle of fish, so to speak. Generaly in lowerlight tanks a depletion of some or many nutrients often does not cause any immediate problems due to the slower growth of the plants and algal colony in the tank. I high light tanks the smallest imbalance or deficency of a nutrient can cause sever health prob;lems to the fast growing plants and give algae the foothold it needs to bloom.

This is evident with BGA(blue-green algae) If you let your NO3 hit zero ppm, you will most likely get it. In high light, CO2, fast growth situations, a small nutrient imbalance can cause a cascading "domino" effect in the nutrient balance of hte tank and lead quickly to algae, poor plant health, etc.

also, allowing a nutrient to run to zero in a high light tank is th equickest way to have everything looking horrible, plants growing poorly, algae blooms, etc. For this reason, generally we give out tnaks more than enough nutrients to sustaine them. Often we limit one in an effort to control the "balance" of our tanks (or to increase the reds of ludwiga repens, for instance) While we may limit it, gemerally we never will let it reach 0 ppm. In an effort to eliminate any build of of nutrients that could eventually become toxic, or build up beyond levels that are needed by the plants in a reasonable amount of time (usually a week) we do large water changes in an attenpt to start "fresh"

Hope that was helpful:)

· Banned
2,072 Posts
Non CO2 tanks do great with no water changes, sometimes for 2 or more years. Discus have bred in some of these tanks.

Water changes takes the guess work out a great deal of possible variables. Ideally, we could add just the right amount to balance the uptake by the plants. Some folks have that balance down or happened upon by luck or a lot of experience.
Some folks are smart and use 2 watt gal+CO2 and a good fish load and get away with less routine dosing. Some methods rely more on the substrate as a source.

But water changes+dosing allows you to know pretty closely without testing what the nutrient levels are except for CO2.

You can easily guess 2-4 times dosing during the week till the next water change.

Some folks can do fine with 2-3-4-5 weeks out without issue.
But most eventually get burned.

But doing a weekly water change is easy and lets you know better what the parameters are.

You can see for your self the difference in your own tanks by doing the water changes vs not.

Try it. I've done it.

Tom Barr

· Premium Member
2,036 Posts
The water change is an easy way to repair the consequence of mineral dosing mismanagement. We wouldn't have to reset the values if we were able to dose only what is needed for the plants to grow at the time.

For example, Potassium is dosed by K2SO4. What happens to the Sulfur after plants take up the Potassium? Remains in the water column indefinitely and building up with every dose. Some people may say there is no evidence to suggest Sulfur causes problems. It does in the long run when levels get high. Sulfur is not an essential plant nutrient, however plants take some Sulfur if present but only as a luxury uptake. Same applies to Cl and Na.

The same issue is with the other minerals we use, MgSO4, CaCl, CaSO4, NaHCO3 and so on.

I can see the presence of the remaining ions on the TDS meter in uS. They do cause a huge increase in water conductivity with each dose that can be interpreted as a water contamination.

Our hope is to find a 'cleaner' dosing system.
KNO3 with KH2PO4 creates NO3 : PO4 : K ratio of 10 : 1 : 6.6 that makes acceptable mineral contents to be taken up by plants completely without leaving anything behind.

For the two remaining nutrients Ca and Mg, I am testing CaCO3 and MgCO3.

I have found by experimenting that dosing daily gives plants a better chance to get the whole spectrum of minerals they need. This way the plants never stop eating.
But when I dose once a week, plants take one nutrient faster then the other nutrients, causing unbalanced ratio, which leads to slower uptake of the remaining available nutrients in the water column.

So far so good, time will tell.


(Can this be moved to On fertilizing... forum?)
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