helenf

I have an idea about this, but would like to learn other people's experiences and opinions. If you have had an experience like this, please let us know what yours was.

I'm comparing two tanks.

5 gallon NPT: Soil+gravel substrate, heavily planted with fast growers (hygros, ambulia, duckweed, elodea, a crypt is growing magnificently, etc). Moderately heavily stocked with a betta, male endlers, a few shrimp, an apple snail. Only circulation is an airstone running slowly. No filtration.

30 gallon coolwater community Gravel substrate, heavily filtered, lots of oxygenation and water flow from the trickle filter, heavily planted with fast growers (mostly elodea and different hygros, plus slower growing anubias and java ferns). Added DIY CO2 and tracer fertilizer. Moderately heavily stocked with goldfish, siamese algae eaters, female endlers and fry, white clouds.

The temperature of both tanks is about 23 degrees. Both always have zero ammonia and nitrite. The nitrate level of the NPT is 5ppm. The nitrate level of the other tank is around 20-30ppm, with 3 or 4 10-15% water change weekly (I change a bucket out every other day, basically).

Now, I think that I feed the NPT less for its volume than the larger tank. I certainly take out much more plant cuttings from the larger tank, for its volume, as the growth in the larger tank is much faster (the elodea, in particular, grows a few cm daily since I added the CO2). But the nitrates in the larger tank are a struggle, whereas they are not a problem at all in the NPT.

Why the difference in nitrate levels? Surely less food + faster growth should mean the nitrate levels in the filtered tank would be lower, but they are anything but!

I do have some ideas, but I'd rather hear from other people first, if you don't mind. So what are your experiences comparing NPTs with your other, filtered tanks?

isu712

I think that the goldfish in the larger tank might have something to do with it. I know that they can add a lot to the bioload of a tank even if it is heavily planted. How many goldfish are in there? And how big are they? That's my best guess.

Red_Rose

I've never had a tank cycled with a filter and to be quite honest, I don't think I ever want one but I do know a bit about the nitrogen cycle.

Your goldfish tank is cycled with a filter so the primary source for getting rid of ammonia and nitrites is from the bacteria colony in the filter media which converts ammonia and nitrite into nitrate which would cause the nitrates to gradually build up. While yes, plants do like nitrates, they much prefer ammonia so the plants and bacteria could be competing for the ammonia.

In your NPT, while yes, there is nitrifying bacteria in the soil, the plants are the ones taking over by removing the ammonia and nitrites instead of converting them which would be the reason why you have such a low nitrate reading in that tank.

helenf

Goldfish do make a lot of waste in a tank, but only because they eat a lot. The amount of nitrogen going into a tank dictates how much there is to be used in the tank (as ammonia, nitrite, nitrate and N2 gas).

So while yes, goldfish certainly add to the bioload in a tank, their presence doesn't alter the "equation" of "I put so much nitrogen into the water as fish food and I take so much nitrogen out of the water in water changes or as plant cuttings, or it escapes as N2 gas, or it gets stored in the plants that stay in the tank".

My understanding is that if everything else is equal, you feed two tanks the same you should get the same nitrate production in each, even if one has a larger bioload than the other.

helenf

This is part of my guess as to what is going on here. I am just very surprised at the scale of the difference. I expect the plants in my NPT to take up more ammonia than those in the filtered tank, but not so much more as to mean I never have to do water changes, in a tank that I'm feeding more per litre anyway. Both tanks have both processes happening (ammonia removal by plants and by bacteria).

Does anyone know how much more ammonia than they need plants are capable of taking up and how quickly they do it?

I am inclined to guess that in the NPT there are more processes going on to remove nitrate from the water, and that this is the reason it is so much lower. But it's a guess only, because I have no actual idea of the relative scales of all these processes. Which is why I'm asking.

Maybe my question could be rephrased like this:

Is the key to nitrate control in a planted tank ammonia consumption by plants plus reduced biological filtration, or is it bacterial processes going on in the soil that convert nitrate to N2 gas?