I've got about 8 guppies in my 55 gallon community tank and evey now and again one will drop to the bottom of the tank for a day or two and then die. This has happened with 4 guppies now and I can't for the life of me figure out why. My tank parameters are good and as follows: Ammonia: 0 Nitrites :0 Nitrates: 40-60 (hard to discern on api kit). PH-7.4 Water temp is kept at a steady 80F and 30%(give or take) water changes are performed weekly. Prime is added when it's changed. Filtration is a fluval406. Bioload: 1 BN pleco 2 small angelfish 3 yoyo loaches 3 kilhuli loaches (definitely not spelled right) 8 fancy guppies I also keep an array of live plants. No visual indicators of any kind of sickness. I don't see any bullying or any signs of it. One day they're happy, the next they're laying on the ground and soon after dead. Anyone have any thoughts?