i have 4 fish in my 5 gallon tank. 2 small guppies and 2 snmall cory aeneus catfish. they have been completely fine the entire time ive had them in the tank, its a new tank and still cycling but i was told that these would be hardy enough fish for it. one of the guppies suddenly wasnt swimming around at all yesterday he was sitting up at the top side of the tank. and this morning i couldnt find him and he was dead in the filter. what is wrong? the other guppy and the catfish all seem healthy. and i thought they would be health. i also tested the ammonia levels and they were 0 yesterday. so whats wrong? and what can i do to ensure the health of my remaining fish?