It sounds to me like they got a parasite or disease maby from the change of water the last time if they were doing fine till that particular water change. I'm not sure what you have your tank heat set on but sometimes if you turn it up about 3-5 degrees until you get to a pet store to get a medication for parasites and diseases, the higher water temp starts to kill whatever is in the water but it won't take it away completely, and when u get to the pet store asl someone what they fell is the best solution, beleive it or not i have almost always used ich treatment for all kinds of parasites other then ick and it helped me, Or atleast call a local pet store owner and ask them what they think may be going on, also check everything and make sure its working orioerly from filter to heater, and after you treat your tank change everything to new inside the filter,like the carbon oe sponge or whatever you may use..good luck Jen
