Catfish Issues?

I had a customer come into the store to return a catfish (alive) and some guppies (dead) because she thought that the catfish may have killed the guppies because the guppies had all died and she'd noticed the catfish constantly swimming up and down from the surface to the bottom of the tank. 
And she thought that somehow the catfish had been the instigator of the others deaths....

*shakes head* 

I doubt it was the catfish.
They're usually not aggressive to other fish. Especially the Cory's. 

But the fact that she put 8 guppies in a 5.5 gallon tank may be part of the reason why they all died.
She was overcrowding the tank. 

But there was also the consideration that guppies...are just really delicate.
They can die so easily. So very easily.
It may have had nothing to do with the tank conditions or the other fish in with them.
It may have been that guppies just weren't healthy enough or something.

In any case.
I returned the catfish
And replaced her guppies...
With some platys.
Though I did my best to convince her to buy less fish because buying a ton again would only kill the fish once more.

I partially succeeded....
So hopefully her new fish do well....

-Sarnic Dirchi