I have 10 gallon freshwater tank. all readings are fine except niTRITES. what is the best way to lower levels?

Question

I set my tank up a few months ago and let it cycle before adding fish. I just had one fish die checked levels again and noticed nitrite level is high- all others are fine. My black molly seems a little aggressive- does this have anything to do with this problem I'm having?

Answer 1

The best thing you can do is an immediate water change.  I suggest about 25 - 30%.  I don't know what you did with your tank before you added fish, but you didn't fully cycle it, or you wouldn't have nitrites now.  Your tank is in the middle of cycling, but should be near the end, fortunately.  Please see the link below on the nitrogen cycle.

The death of your fish may very well have been due to nitrite poisoning.  It's hard to say if the aggression being shown by your black molly is due to the high nitrite levels in the water, but it could be the case.  No doubt it is causing the fish stress, which can show up in a variety of different ways.

Answer 2

Her aggressiveness would not have anything to do with it. Do a daily water change of 15% for a week, monitoring the levels every day. This will help reduce them. And by adding salt you will not only benefit mollies, since they are an actual brackish fish, and you will detoxify the nitrites with it.

How many fish are in the tank? For that size of tank, I would only have 4-5 mollies at the very most. Overstocking or stocking too quickly can also cause nitrites to rise.