I have a 60 gallon community tank, properly established for around 3-4 months and there are healthy(apparently) swordtails along with other peaceful species. They are dying one by one. I've got 4 dead in the past 1 month. I also got 2 fry born 2 weeks back. Nitrates are 6 ppm, PH, GH, Ammonia, Nitrites all ok. No spots, redness or any identifiable physical or activity change. I feed them pellets, flakes, live bloodworms, brine shrimp. No other species seems to be affected. Only the swordtails. Male-female ratio is 1:3. I can think only of overfeeding, bacterial infection (possibly through bloodworms), maybe bad batch with low immunity or something. All of them were okay initially. One female had one or two pea sized discoloration spots. It was probably the one which gave birth. Not sure. Please help me find the cause and remedy for it. Thanks.