I set up a 55g in December and so far I've had 26 fish in there, and 12 have died. I didn't really achieve cycling until April, and after that things were looking great. But then in late May, 2 cories expired, and in June a platy and a paradise fish have kicked the bucket. None had any visible signs of disease, and only one showed suspicious behavior (hiding out in an ornament while it's brother was very present). I hate to acknowledge this, but I got a little lax on the water changes. I had been changing roughly every 2 weeks, and was adding fish steadily (2-4 fish every 2-3 weeks). That may have something to do with it, I realize. The last 2 deaths occurred within days after a water change. Since mid-April, pH has been steady at 7.6, ammonia has been usually 0 (twice it was maybe as high as 0.25), nitrite 0. The nitrate had been 0 from day one up until mid-May, and it is now at 5.0. In the original setup I used shower water but within a couple months started using R/O from a LFS, then transitioned to Glacier (which I've been using exclusively since early April). Stock is now: 5 Buenos Aires tetras 4 platies 3 cories (2 emerald, 1 Schwartz) 1 paradise fish 1 zebra danio Filters: Cascade 1000 canister filter, Slipstream 75 (HOB), Marina 20 (HOB). Temp: 75-76 degrees (has been steady for months) Food: Flakes and sinking pellets. I have also used frozen food but I noticed that I always seem to have a fish death after using it, so stopped using it after the May deaths Picture is from 5/20. Nothing changed aesthetically since. If I had to guess, I'd think it's the nitrates, but I haven't read anything to indicate that 5.0 or less should be lethal (I realize ideal is 0). This venture has been soul-crushing. I feel terrible b/c I'm sure I'm doing something to kill them off, but I just don't know what it is. The fish are from various stores. In the beginning I went to the chain stores, when those fish didn't do well I switched to specialty stores and the fish were much better, but now those fish are dying, too.