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I have since last Wed gotten a Nitrite spike, rising from less than .15 ppm to 3 ppm.  I think it may be due to over feeding, although I don't find much food residue on the bottom of the tank.  Not sure how the spike is being created......... I am losing 1 to 3 fish a day form what I guess is Brown Blood disease.  I have stopped feeding, last feeding being this past TUesday.  Nitrite levels still running around 3 ppm.  Anyone have any ideas how to reduce nitrite levels quickly?  Or what might be causing the Nitrite spike.

Ammonia level is .5 ppm


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Do a 20% water change with dechlorinated water, NO MORE than 20%.  Be sure that the pH and TEMP are the same as what's in the tank of fish otherwise the fish may succum from shock.  Add more plants or harvest some fish into another system or the freezer.

It's hard to tell without more info, but you're probably close to hitting the nail on the head with the overfeeding idea (just because there aren't any leftovers doesn't mean they didn't eat enough to overwhelm your biofilter). You may be seeing a secondary nitrite spike resulting from your biofilter adjusting to the increase in bioload (think of it like another shorter mini-cycle as the bacteria multiply to handle the load). You can try partial water changes with pH/temperature adjusted water to bring down the nitrites and see if that reduces the stress on the fish.

When you start feeding again, measure out exactly how much you give them in your first feeding. Give the same amount each time for a few days and watch your water parameters. If your water quality stays steady, slowly increase the amount you feed them watching for any signs of changes in water parameters daily. If you see an ammonia spike, back off on the feeding. If the ammonia stays steady, then you can feed ever so slightly more the next day. In this way you can slowly increase the amount your bioload can handle without overwhelming it and starting a mini-cycle.

Good luck!

thanks for the replies......I have already begun to flush 400 gallons into the system, and I will see what Nitrites are tomorrow.  I think I read somewhere that removing the covers on the troughs can also burn out the Nitrites.  I will do some research this evening and see if I can confirm that.  If that turns out to also be a potential solution, I will take the floats off one of the 16 foot troughs.

Thanks again.


Thanks for all the support.  I have added water, and removed rafts to so that the sun can reduce the nitrogen levels.  I have gotten down to about 1 ppm nitrites.  I am still not certain what caused the nitrite spike, and I am not convinced that over feeding was the cause.  When I cleaned the tank, I found very little food laying about. And ammonia levels have never gotten higher than 1 ppm, and currently are not detected at all.

I have not lost a fish in 2 days, and I am hoping i am past the crises.  I have read about "Brown Blood" disease in channel cats, and it is 100% due to excess nitrites in system.  Research has delivered a solution to Brown Blood disease, in addition to reducing nitrites.  It is suggested that a salt level of about 50-60 ppm of sodium chloride will enable the cats to live a healthy life with a higher nitrite level.  Research also points to sodium chloride levels below about 75 ppm to be harmless to plants.

It seems it would be good management to carefully monitor and maintain a level of salt about 60 ppm in the AP system.  Anyone here doing that?


I'm not doing that currently, but I have read many, many discussions on this forum about salting proactively to up to 3 parts per thousand for catfish (they apparently are more sensitive to salt than some other types of fish and cannot tolerate higher levels than that except for baths). You would not be alone if you chose to salt your system proactively. Salt does help fish get through stress events and nitrate spikes and diseases like ich. It also helps with osmoregulation. I haven't done it in my system so far because I haven't needed to (knock on wood) and I don't have a way to monitor the salinity in my system to ensure I don't overdo it with any additions after the initial dose. You apparently won't be able to grow strawberries though ;). Good luck!

On my way to Pentair to buy a chloride test kit, and additional Nitrite/Nitrate test kit.  

What kind of covers are being used on your fish tank?  I have plywood as cover, (not PT)..... is there anything in the plywood that could cause a Nitrite spike?  I have never had ammonia levels higher than 1.0 ppm and usually run around .5 ppm. Everything I have read indicates that the Nitrites feed on the ammonia... it would seem to me that I would need to see some increase in ammonia to generate the nitrite spike.

Very confusing!


Here are a couple of ideas/opinions which you can totally blow off it you want to:

Somebody please correct me if I am wrong, but I think you need a salt refractometer to measure salinity. Something like this. I'm not sure a chloride test kit does the same thing.

I have a growbed over my fish tank and a tarp over my sump tank. I have tried to avoid wood directly over the tanks due to the possibility of chemicals or tree sap exposure to the system (both can be toxic to fish). I don't think that plywood would increase your nitrites but it might contain some nasty chemicals that you wouldn't want leaching into your system. If it were me, I would try to cover the plywood in some way on the side that is exposed to the system, like with a  tarp or something, or avoid the plywood altogether.

How often do you test your system? I test my system water daily for pH, ammonia, nitrites, and KH and weekly for nitrates. If you don't already have an API Freshwater Master Test Kit, I would go with that since it allows you to test all of those parameters except KH (you can buy a kit for KH/GH separately). Test daily for a while to get a feel for what's going on in your system especially relative to how much you are feeding. I am thinking that over feeding with infrequent testing is probably the culprit here, but that's just a guess. Only you know for sure.

Check the forum for information on how the nitrogen cycle works in an AP system. Nitrites don't feed on ammonia since nitrites are a chemical byproduct of the nitrogen cycle. From my understanding, the cycle goes something like this: (somebody correct me if I am off base) Ammonia is the waste product of the fish which gets "eaten" by nitrosomas bacteria and converted to nitrites as their waste product. The nitrites then get "eaten" by nitrobacter bacteria and their waste product is nitrates. Nitrates then get "eaten" by the plants and your water is "clean". If you have more ammonia present than you have nitrosomas bacteria available to "eat" it, you will have an ammonia spike. The nitrosomas then multiply to accommodate the new level of ammonia "food" converting all the ammonia to nitrites, but now you have more nitrites present than you have nitrobacter to "eat" it. Now you have a nitrite spike which lasts until the nitrobacter multiply to the point where they can process the new level of nitrites into nitrates. I am thinking that you had this happen in your system and you are at the nitrite spike phase which will end eventually (or may have already if your fish are no longer dying).

Salting seems like a sound idea to ease the stress on your fish along with the steps you are already taking to lower the nitrite concentration. Just don't overdo the salt with catfish (3 parts per thousand max apparently). When you start feeding again measure the amount you feed and test your water daily. You will start to see a pattern in your system that you can use to increase or decrease your feeding amounts based on. I can't think of anything else that would cause a nitrite spike than the above.

Good luck!


Thanks for your thoughtful reply, and your description of the ammonia/nitrite/nitrate process is exactly as I understand it.  I have made several inquiries, and it is the Chloride in salt that improves the tolerance of Channel catfish to withstand the higher levels of Nitrite that I am experiencing.  It is a common solution for pond grown catfish, and the recommendation is for chloride levels of 50 ppm that is recommended.  In fact, they recommend the 50 ppm as a maintenance level to prevent Nitrite contamination to the catfish.  Chloride levels above 75 ppm can become toxic for certain plants, but is not generally considered dangerous in an AP system at 75 ppm and less.  Higher levels can become problematic.

As I understand the chemistry, there is a difference between salinity and chloride......I am concerned with chloride to help the catfish breathe in an elevated nitrite environment.  Salinity is a measure of total "salts" in an environment, while the chloride tests just for chloride.  

The longer term issue is that continual additions of salt can promote sodium accumulations, which can be trouble, so I will have to learn to manage Nitrite spikes as they are occurring.  In the meantime, I am adding 1/4 lb salt to my system of 2,000  gallons, which should help my channel cats breathe.

Iam looking for a nice sheet of lexon that I can use as a cover for the fish tank.

Thanks again for your help.


I am not sure I was all that much help, but it's nice to have people to bounce stuff off of. Good luck!


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