Nitrate and volume of water (TAN)


I am currently on a two-year full-time education about aquaponics. One subject I discuss with my fellow students right now is TAN and Nitrate.

If you have a system and you feed 20 kilos a day, and the plants can remove the waste/nitrates from 15 kilos of food, and you remove the rest from the sludge filter, you will have a nice balance of, let’s say, 50 mg/liter of nitrate.

What happens if you double the volume of water in the system but maintain the same kilo of feed per day and the same volume of plants?

The nitrate mg/liter will, of course, change, but how will the plants react?

Are the plants so efficient in taking up nutrients that it doesn’t matter if it is a little more diluted in the water?

Do you have to stop taking out nutrients from the sludge filter until you have 50 mg/liter again?

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Hey Tomas! My name is Cori. I’m the current communications director for the Aquaponics Association.

Great questions. I think plants have an optimal range of nutrient concentrations for the most productive growth. If you go too low on nutrient concentrations, the plants could slow in growth or compete with each other. An experiment might be helpful to know what would happen in a real system.

Hopefully someone else can add some input.

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Hope so.
My thought is that even if you have more water in a system with working bacteria the plants will not really start using up the nitrate before it is at a certain level, mg/liters, so it makes no big difference if you have 20 000 liters or 60 000 with the same amounts of fish and plants it just takes some days extra for the level to get let say 70 mg per liter and then the system balance itself. So, the most important is how much feed you give per day to get the fish growing and the plants happy and a biological filter that can take care of it.

Bigger volume of water should make the system more stabile so if you have the opportunity to make bigger thanks and have more water in the DWC and the sump it is just a plus that makes the fish more happy and the PH and temperature more stable and changing slower.

That is my thought and not all that I discuss this with, I am almost scared to ask my teacher because I don’t really think she/he have the knowledge at all.

If it does not mater can you for example calculate the amount of nitrogen by just kg/food per day, kg/fish and not take in account at all liters of water?

I would love if there is some people to feel the time to take a zoom or teams discussion about aquaponics with me I have ideas and would love to discuss them with aquaponics put there.

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Hi Tomas,

This is a great question/thread.

If you double the system volume but still feed the same amount/have the same amount of plants within the system then you will just need to account for the dilution with doubling the volume. Essentially, you would need to build the nutrients up to a certain point within the water column to where the nitrates are the same concentration as before and then continue feeding as you mentioned.

Another aspect is that if you double the volume within the system than you will need to account for a larger pump to move the water around within the system the same velocity. Additionally, you may also need to increase aeration as you are now dealing with a larger volume of water and likely more biological surface area.

I’m curious as to where this question comes from… What sort of situation would you be in that you would be doubling the system volume but not increasing plant or fish production? From an economical standpoint if wouldnt make sense to double your volume but not increase plant and fish production.

Anyways - hope this helps!


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Hi JD.
The question is in a discussion in our school I am studying a two year program that are named aquaponic engineer. So the students including me have a lot of questions and it is heated discussions about fish health, fish welfare, fish quality, mudd flavor (geosmin) clearing tanks, active carbon, bio carbon, and so on.

Me myself am going to start an aquaponic system in Tanzania. I am working on two designs, one small backyard off grid system for Clarias and easy growing plants like amarant. One bigger system where I want to concentrate on the fish side, also Clarias.

The big one will be big tanks with a total watervolyme on 80 m3, three double DVC 330 m2, and one smaller media bed with an ebb and flow system, a sump and some filters of course total of 160 m3 water. I would like to stock around 50-100 kg Clarias per m3 in the tanks of 80 m3. (produce around 20 to 40 tons a year depending on stocking density)

I could stock 200kg/m3 but will have 50-100 kg per m3, this will still produce more nutrients than the plants need, and I am thinking of how it affects the plants and fish. Even if Clarias can live very crowded I see that they do like and thrive when not to crowded. I am thinking of using swirlfilters to take away the extra nutrients/sludge and mineralizing it and use for hydroponic system that a nearby farm have or maybe even make bio carbon and load it with the sludge and use it as fertilizer for better soil composition. That way I could make the aquaponic system a carbonsink!

Right now I am helping breeding salmon, trout and sturgeon for an NGO in Sweden, but I would like to go back to Tanzania to my house there and leave cold Sweden (o:

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A nine month old sturgeon getting a transmitter fitted before realesed in to the wild in a rewildering project to bring the sturgeon back to Swedish rivers.