I have a 55 gallon tank which has been set up and running for several months. It has undergravel filtration as well as a "hang on the back " filter system. Both seem to be working well. I am having a problem with fish dying within 24 hrs. of being put in the tank. I have had the water tested at two different pet stores and the water tests out fine, ammonia, nitrates, etc. all at good levels. I am not new to keeping tropical fish and have never encountered a problem of this type. My latest experience was with feeder goldfish purchased last night and brought straight home from the store. I let the bags float in the tank for about 45 minutes before releasing the fish. They were immediately active, showed no signs of stress and ate readily after an hour or so.....today they are all dead. I would appreciate any help anyone can offer as I am at my wit's end......I've had goldfish live for years in nothing but a fish bowl!