In my 55 gallon tank I have 17 guppies, 11 platys 15 neons and 3 catfish..... is this ok or overcrowded????? Please help

going by the "inch per gallon" rule.. you have about 90 inches of fish in there... thats quite crowded.

I think it's overcrowded. That's 46 fish. If they were all under an inch long, and the tank were established (with biological filtration), those fish could posssibly be okay. But, the platies and catfish (what are they?) will be more than an inch long (at least some day) and perhaps the guppies too. Plus, the guppies will probably breed. If this is a new tank, the tank is probably having a high ammonia spike. If it's established, I would find an additional tank to which to move some of the fish. Good luck!