I would lean towards low DO,seems med to large catfish can be the first to go. I would "slowly" start up the aeration system and also get some splashing at the surface at the same time. A 1 hp pump can produce a lot (possibly too much ) air depending on what type stones you have, you can also bypass some of the air to the stones to slow things down. The surface aeration is of great value if DO is the cause, DO NOT turn the bottom aerators on 24/7 as you will lower your DO in the upper water before you raise it. If you know the specifics of you system (pump type, model Gast 1423 is a 1 hp rotary vane for example,4 or 6 stone plate etc) I probably have the lifting specs on it and can advise you You are starting to get some cool nights in your area as we were 53F in NW Ohio this AM , this is going to sink or push down the higher surface DO that many other fish may be depending on.I dont think treated lumber was the problem as you would have more than just the cats gone. I have had treated in mine since I rebuilt the pond in 2002 with no problems.Consider acting now as this could be the just start of things
