OK, so I set up a tank this past summer and everything has been fine up until now. My mother became extremely sick (cancer and a spleen removal) and had to have trauma surgery just before Christmas. We weren't sure if she was going to make it. Please don't think me a bad fish keeper it's just that we were spending three hours per day at our house sleeping trying to help get everything else taken care of. I usually clean my tank once a week and do a large water change and have had absolutely no problems up until my mother became ill and I had to skip one week. I also missed changing out the carbon filter and the sponge filter inside my AquaClear filter. When I went to clean out the tank my husband noticed that just one of the platys (platies) was acting odd. She is the smallest fish in the tank and has always seemed delicate. I could find no spots on her, nothing strange about her ordinary appearance and it was very late so I thought she was just tired. Fast forward to this week I went to do another water changing and cleaning and noticed that she is definitely acting very odd. She appears very weak, she is laying on the bottom of the tank. When I put food in she waits for it to float down near the bottom and then will swim ahead just a little to eat. She is moving her fins very fast but not really getting anywhere (the fish shimmy?). The closest thing that I can find to fit her symptoms are cotton-wool disease. But I dont' see any cottony white patches on her. She is yellow and black and her color does appear to be dull and slightly grey in places but it's very difficult to tell because of her natural color. All of the other fish are very healthy in appearance and in behavior. I noticed this late last night as I was cleaning out the tank and haven't had time to do water testing although I do not think this is ammonia poisoining, etc.. because none of the other fish are affected and there are no red traces around the gills, etc... I really don't want her to die and I don't know what the best approach is. I am stuck at work until 5 p.m. and then I am going to stop by and see about getting her some medicine. Should I quarrantine her in a heated fish bowl and treat her with medicine or would moving her create too much of a shock? Should I treat the entire tank and if so, with what? Please, please, please help!