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Discussion Starter #1
In my 10 gallon "el natural" I have an ongoing problem with trace levels of nitrite (clearly not zero, clearly less than 0.25ppm, which is the lowest marker on my API drop test). It seems to be a very persistent problem that I don't understand the cause of.

The tank is heavily planted and lightly fed. The plants are growing fast, and I have to prune the various hygros every week. I scoop off duckweed every few days to feed my goldfish too.

Usually the nitrite is just a trace, barely enough to darken the test tube from the sky blue it should be to a darker blue. But today there was more than a trace, so I did a water change. This afternoon the nitrite is higher than it has been for weeks - around 0.25ppm. This is bad!

One thing I did today that I don't always do is squeeze out the sponge in the little filter I have in the tank to create a current and give some mechanical filtration. The sponge is way too small to bio-filter the whole tank (it is a hagen elite mini filter, designed for tanks up to 3 gallons, so 1/3 of the size I'd need to filter this tank). The sponge today was very clogged with muck and I gave it a good squeeze and put it back.

Obviously the sponge in question is cycled. So by squeezing it I might have removed some bacteria. But I don't understand why doing this would cause a nitrite spike and not an ammonia spike.

I don't understand why there is this persistent (months of it) trace nitrite in the water anyway.

One other thing going on with this tank, that I'm not sure is OK, is that when I very lightly vacuum the gravel, bubbles of gas come up from the substrate. I think I'll make another topic to ask about that one.

What can I do to get rid of the nitrite that keeps sticking around in this tank?

I've added 1 tsp of salt already.

Should I add a second airstone?

Should I remove the little filter altogether, so there is even less sites for bacterial conversion of ammonia to nitrite? Then there will be less current. I am worried that just removing the sponge might mean little shrimp get sucked in the slits and smashed up by the impeller. Maybe this is an acceptable risk?

Any other ideas? This is driving me nuts!
 

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Couple of thoughts, dunno if they are worth anything.

Double check the test kit. Perhaps you are getting a false reading. Do you have other tanks? Test some tap water. Take a sample of the tank water to a store and have them test it.

The bacteria that remove nitrite grow more slowly than the bacteria that remove ammonia and produce nitrite, so this might be part of what is going on. Something is making it just a little harder for the bacteria.

Can you reduce the source of nitrogen? Less fish food, do not add any nitrogen fertilizers. Perhaps the plants can better keep up with nitrogen export.

There are other bacteria (and other microorganisms) that live in the soil and run the nitrogen cycle 'backward'. With the gas you are seeing I think this may be happening in your tank. If the gas does not smell like swamp gas you may be seeing nitrogen gas (N2) escaping, and part of this cycle is resulting in NO2. It is my understanding that these bacteria live in a lower oxygen zone than the nitrifying bacteria that turn ammonia and nitrite into nitrate. Perhaps stirring up the substrate in these areas (as suggested in the other thread- a little poking daily) or adding Malaysian Trumpet Snails to do the stirring for you would help. Maybe reducing the depth of the substrate in this area would help, too. Keep the sand for the Cories, but remove most of the soil where there are no roots.

What are your test results for ammonia and nitrate?

The generally accepted dose of salt for Brown Blood Disease is 1 teaspoon per 20 gallons, so you can back off a little on the salt.
 

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Discussion Starter #3
Thanks for your thoughts. Please keep coming with new thoughts, because I am stuck!

Some comments to your questions:

- the nitrite test appears to work and it certainly measures zero on the water from my other tanks and the tap water.

- the ammonia level in this tank is always zero (measured regularly, since I've had the nitrite problems). The nitrate is around 10-15ppm.

- If the nitrite-consuming bacteria are simply being slow to catch up, how come after weeks they still haven't caught up to give me mostly zero nitrite (this is talking about the persistent trace levels, not the current spike, which may be caused by something different).

- your points about the substrate seem plausible and I will certainly try more poking (in the morning, it's nearly midnight here, and fish asleep and so should I be) and see whether that helps. I can't get malaysian trumpet snails here in Australia, as far as I know.

- I am not adding any fertilizer and I tried halving the amount of food, and this did not make the trace nitrite go away. I don't think it's the food that is causing it, but I'm not certain of anything.

I won't add any more salt and will remove that with water changes over the next few days. Thanks for the dosage, I never know how much is enough to remove/reduce the risk of my fish and shrimp getting ill from the nitrite I am inflicting on them, and I worry about that.
 

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The nitrate is around 10-15ppm.
Here's the problem source. Bacteria in the substrate are converting nitrate to nitrite. "Nitrate Respiration" (my book, page 65) is a very common bacterial process. Whenever oxygen is gone, which is inevitable in soil substrates, many ordinary bacteria quickly switch to using nitrates "to breath". The result is nitrite. This simple one-step process is much more common than the complete conversion of nitrates to nitrogen gas ("denitrification").

You say that the substrate is bubbling. That means the bacteria are very active. Nitrate enters the anaerobic substrate, and bacteria there quickly convert it to nitrite. Its that simple.

I would try to reduce nitrates in the water by changing the water and discouraging biological filtration. Let the plants take up the ammonia.

Your tank probably started out with an excess of nitrogen. It could be from the substrate or previous fertilizer additions. Eventually, the excess nitrogen works its way through the system. In the meantime, especially with the salt additions, your shrimp should be fine.
 

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Discussion Starter #5
Hmm, I'll try turning off the small powerhead/filter I have in there, which has a sponge in its intake. That's the only removable biological filtration I have in the tank. Maybe it'll make a difference. I'm a bit worried about the possibility of causing an ammonia spike, though. Maybe I'll just have to risk it and see...

Thanks for the comments.
 

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Discussion Starter #6
OK, I removed the small powerhead a few days ago. No ammonia spike. Nitrite low for a few days.

This morning, for no reason I can understand, the nitrite is back at 0.25ppm.

I can absolutely believe that bacteria are converting nitrate to nitrite in the tank. What I don't understand is why that conversion would be so uneven as to create actual nitrite spikes. Surely there should be more bacteria growing to convert the nitrite back to nitrate, and the processes should balance.

I have not been feeding more than usual, if anything a little less.

There is as much oxygen in the water as usual (running an airstone 24/7, and have been for weeks).

Any suggestions as to why this keeps happening?
 
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