20 gpm is a lot of water, but it's not a huge amount of water wrt a 1 acre pond. Most well water is OK for fish to live in, albeit it usually has no oxygen when it comes out of the ground. I don't think 20 gpm of low oxygen water going in to 1 acre would be the problem that killed the catfish.

New angle. Can you describe your aeration setup and schedule? 24/7 or what?

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
[Linked Image from i.pinimg.com]