Ponds are more exposed to parasitic diseases than tanks are, because many parasites require another host, typically a bird or a live invertebrate, to complete their life cycle. Therefore you don't get those diseases at all in aquariums (provided that the fish have never been kept outside). There is also the possibility of exposing tropical fish from another country to your own local diseases, which they have less resistance to. But it would be hard to say how great the risk from using a pond sponge was.
Josh, ammonia is always bad, but to be honest the gibbiceps looks underweight to me anyway. Had you just bought him? They are often not properly fed while in the shop unfortunately. Sorry you lost him