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The Mystery of the Disappearing Nitrogen


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I have a mystery on my hands. I have a new setup that is heavily stocked (6 Honey Gourami and 6 Panda Cory in a 12 gallon long) enough that it should be producing nitrates. It was fish-less cycled and the fish have been in for two weeks (Gouramis) and one week (Cories). The fish are all eating well and producing waste. I am experienced in fish-keeping so am confident that I cycled the aquarium properly and kept the ammonia load high enough to sustain this load of fish. Just to be safe, I added Turbostart when introducing the Gouramis.

Here is the mystery-- the water tests as 0 ammonia, 0 nitrite, and confusingly 0 nitrate. The aquarium is planted but only with Anubias which should not absorb nitrates this fast. I have tried fertilizing the aquarium with Easy Green and testing the water the next day. Each time the readings are the same-- I dose Easy Green to 20-30 ppm nitrates and it is back to zero by the next morning. I have tested using both test strips and API liquid tests multiple times with the same results. I don't to want to keep dumping more and more Easy Green in until I know what is happening in case something other than the plants absorbing the nitrates is going on because I am worried about causing a buildup of phosphate, iron, etc...

My possible theories (though none seem likely):

1. There is enough anubias to rapidly and completely absorb large amounts of nitrates even though the light is relatively low and there is no CO2 injection.

2. The excessive filtration on the tank (a Fluval 306) containing large amounts of Biohome Ultimate (4 pounds) is doing the complete cycle that the media is advertised to do. The filter has only been running for 6 weeks, though, which is far less than the 4-6 months PondGuru quotes for establishing anaerobic bacteria to consume nitrates.

3. I have no idea???


Edited by IanB
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I have no answer for you, but I am very curious about the same thing. My tanks are running 0/0/< 5ppm for over a year. Heavily stocked, full of anubias and moss, low light, no CO2. Light filtration. Thin layers of sand or gravel for substrate.  Should be full of NO3- but aren't. Hope someone can explain where those nitrates are hiding. 

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My guess is that you have a strong colony of denitrifying bacteria! They don’t just live in special filters—they also live in the anaerobic areas of the substrate. According to this article (http://www.aquaworldaquarium.com/Articles/TonyGriffitts/AnaerobicBacteria.html) you only need 1.5 inches of sand to support an anaerobic bacteria colony (depending on how coarse the sand is). And apparently having cories gently nuzzling the top layer of sand helps the nitrates get down to the anaerobic spots.

Denitrifying bacteria will take nitrate (NO3) and turn it into nitrogen gas (N2), which can diffuse through your water into the air in small amounts, or bubble up to the surface in large amounts.

That’s what it sounds like to me, anyway! It seems like you’ve hit the jackpot in terms of robust complete cycling.


Edited by Hobbit
Being more thorough :)
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