New fish, next day its dying... why? please help

[Grimpatches14]

Hello everyone, well yesterday i bought a new fish for my tank. All i know is that it is a catfish, i dont know what kind it is because the pet store hadnt gotton any tags for that fish yet, but since i knew the worker they gave me one anyways. Today i came home about 30 min before this post and i see this catfish laying in the tank, not moving at all, on its side, breathing pretty heavily. one side of it is a little red on the skin, but just on the back half, can anybody tell me whats wrong with this fish? if so, how can i fix it?
Ps. can anyone tell me what kind of catfish it is too?
Thanx

 

[Pyramid_Party]

Porthole Catfish

Is your tank cycled, whats the water parameters?

 

[Grimpatches14]

its an 85 gallon, i have a lot of other fish in there and there have not been problems with the fish, i havn't cycled it for about 2, maybe 3 weeks now, but it does have a nice biofilter as well. and sadly, the fish died today. also, they don't need brackish water or anything special like that do they? are there any special food requirements for them?