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Nitrite increasing


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I have 2 tanks set up for quarantine. A 20 gallon plastic tub and a 10 gallon tank.

both tanks had fish added within a day or two of each other.

The 20 gallon tub currently has 14 green neon tetras.

the 10 gallon has 7 golden topminnows.

Both tanks had cycled filters. Both tanks were treated with quarantine trio of meds and 1 tablespoon salt per 3 gallons of water and allowed to steep for a week followed by twice weekly partial water changes and carbon filtration to remove meds and salt.  

after several water changes I added floating plants to both tanks to provide fish more cover/enrichment.  They like swimming up amongst the roots.

Both tanks developed nitrite rises at the 2.5 week mark.

I test quarantine tanks daily until fish are transferred to final tank.

I started water changes on both tanks and re innoculating for beneficial bacteria.


10 gallon tank is fully resolved after 3 days.


the 20 gallon seems to be getting worse.  A1D7A97E-2FBB-4BB8-9ACB-9AEAEBE58081.jpeg.18cf3c5945dee81dc897d6122e47e16c.jpeg

The vial on the left is after a water change at 6:00 last night.  The one on the right is at 4:00 this morning.

I get up at 4:00 AM every morning.

No measurable ammonia either tank at any time, and both tanks have bare bottom fairly clean from vaccuming.

I wouldnt expect 14 green neon Tetras to produce this much Nitrite in 20 gallons in 10 hours.

I have two box filters and 1 large sponge filter.

The fish have not been fed for 24 hours.

This morning after doing another 75% water change, I added a powered double sponge filter from a fully cycled tank.

Saturday both tanks are scheduled to have second treatment of Paracleanse.  I am hoping Nitrites are resolved by then as I would prefere to not have to do water changes once I dose with Paracleanse.


floating plants all look healthy and vibrant?  Fish look to be healthy and happy but it is a lot harder to tell just looking down into tub.


I find it rather puzzling.  I am glad I test quarantine tanks daily so that I caught it before finding floaters.

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It is important to assess whether you are also seeing nitrates. Have you tested for those? 

I'm not a professional . . . but I'll try to armchair quarterback this:

The nitrogen cycle begins as fish release waste / plant matter and foods decay / dead fauna decompose -->> releasing Ammonia.

There is one colony of bacteria -- nitrosomonas -- a gram-negative bacteria that converts ammonia to nitrite. It would appear like your colony is well.

There is a second colony of bacteria -- nitrobacter-- another gram-negative bacteria that converts nitrite to nitrate. This colony, I am afraid, may not be thriving.

The cause, I suspect, is that the broad-spectrum antibiotic you used knocked out the nitrobacter colony. This is more of a common problem than aquarists think, especially if your QT tank is a limited volume. 

Best course of action would be to add a new, cycled sponge filter once your course of treatment is over. 

You can also add FritzZyme 7 as prescribed with every water change until your nitrites settle down and nitrates build up.

Hope this helps!

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Well, I am at a loss.  0.5 ppm nitrite again this morning….


so 12 hours yesterday during the day nothing, and 12 hours overnight, 0.5 ppm…




another water change, removed all floating plant matter,  who knows…



meanwhile the other quarantine tank, nothing….

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On 8/15/2022 at 7:00 AM, Fish Folk said:

The cause, I suspect, is that the broad-spectrum antibiotic you used knocked out the nitrobacter colony. This is more of a common problem than aquarists think, especially if your QT tank is a limited volume.


Going forward, dosing meds in food, especially antibiotic, will alleviate this problem going forward.

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What I find confusing is that Nitrite stayed zero until weeks after course of quarantine trio was completed.  On day 7 of treatment I did a 30% water change and added a large box filter pretty full of activated carbon and a bit of floss to adsorb meds follwed by another 30% change 3 days later and another 4 days later.  Yet nitrite remained 0 ppmtill about 2 weeks after dosing was completed.


this happened both in a 20 gallon tank with 14 juvenile green neon tetra and a 10 gallon tank with 7 Golden Topminnows which presumably would have a larger bioload.  But the 10 gallon tank never progressed beyond 0.5 ppm and resolved with reinnoculation while 20 gallon has not.

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