Sick Glass Catfish

[youngster54]

I bought 3 glass catfish about 3 weeks ago. I have a 55 gallon tank with 2 zebra danios, a algae eater, and a chocolate gourami. Last week one of my glass catfish was "bent" and could not swim. I tried to help him up with the fish net but he would just sink back down to the bottom. The next morning he was dead. And yesterday another glass catfish could not swim. He was not "bent" at all but could not swim. He was laying down at the bottom of the tank swinging his tail but could not go anywhere. Also it looked like he was on his side trying to swim. Sometimes he would get off the bottom of the tank but be would then sink back down. He then died this morning. All the other fish are fine swimming well. I am afraid the next catfish will have the same problem and die. Please help me. I don't know what to do.
Thanks