Welcome to AA
Did you cycle the tank? Sounded like you just put water, dechlor, and then added fish without cycling? If you didnt, read through the articles in my signature on cycling. That will help you out alot. Also, if i understood it correctly, you are planning on getting an undergravel filter? I would not recommend that. I would recommed using only an HOB
(hang on back) or a canister filter, more realistically an HOB
on a 10g... Also, do you know what kind of catfish it was?