So I've been gone a while. Well, today I need some help. For the last month or so, I've had nitrite. Ammonia is 0PPM, nitrate is 5PPM, but the bloody nitrite won't leave. I constantly have around 0.05PPM nitrite, and for a few days it looks like it is slowly going down, then it'll shoot straight back up to 0.05, then over the next couple of days it will start to move nearer to 0.1PPM, then it'll start going down again, rinse and repeat. I've tried putting in more TSS, but that did nothing to solve the problem. I've ruled out the possibility of the fry constantly growing putting stress on the biological filter and it can't keep up, as my ammonia levels have consistently been spot on throughout this. I'm not overfeeding, I'm very conscious of how much I feed my fish, so that also can't be the cause. I know that 0.05 to 0.1PPM isn't lethal by any means, but I am aware that any nitrite present will stress the fish and make them more susceptible to diseases, and it can irritate the gills and trigger an inflammatory response. This is already happening. My neotenous female (yes, that actually happens, I have a neotenous guppy) has been coping well (she has been in the tank since the start, so she has been exposed to nitrite for a while now), but the long-term exposure to nitrite is clearly catching up to her, because her gills are very red-pinkish and look quite inflamed. I'll be happy if she lives to be a year old. My other fish aren't doing very well either. My yellow micariff male seems to be developing toxin-related swimbladder disorder, my snakeskin's (who is actually a sunset micariff who has started expressing snakeskin patterns for whatever reason) fin-rotted tail has for the most part stopped healing (which is a sign of prolonged stress) and one of my red tuxedo females is just constantly sitting at the bottom, and when she isn't she is either coming to the top because she thinks she is being fed, or she is sitting in a corner doing nothing, which if I'm not mistaken is a sign of severe prolonged stress AKA depression. I don't think it is a disease because she has no other symptoms whatsoever. I am really not sure what to do. It's clearly affecting my fish and sooner or later some serious disease will take hold which I am powerless to treat because iirc you can't treat tanks with fry in them because they don't have the scales (or haven't fully developed them yet) that protect them from skin damage from the medication. I hope I'm wrong in saying that but it's probably true. The only thing I can think of at this point is taking a few of the adults out to release some stress from the biological filter which may let it settle into a cycle, because clearly for some reason the bacteria that converts nitrite into nitrate has an unstable population, and something is killing them or is depleting them of nutrients. My dad suggested it could be an over-abundance of some sort of metal or alloy in the water, since my water is pretty hard as is and for whatever reason I keep the mineral supplements in the filter, and I don't have carbon in the filter so nothing is breaking it down. I thought my dad's idea had some plausibility to it, so I put the carbon back in the filter to see what happens. Nothing will probably change but it's worth a try I guess. If anybody has any more ideas of what could be fvcking up the biological filter so badly then please tell me, I'm running out of ideas.