Thanks all. It must be the fish. All other factors seem ok. I think
I'll add a small light to attract bugs.
My youngest and I got called to the neighbor's pool last night, the frog
"Love Shack" as the call it. They had 15-20 tree frogs already netted.
By the time you walked around one time collecting them there were
already more taking their places. We took the 15-20 home (that was fun
as they were in a small garbage can with no lid) and put them in the
pond. They were instantly singing. Some actually were singing on the
way home. The variety of calls from one species of frog was amazing,
everything from your standard tree frog squawk to monkey like sounds.

I'll add the light tonight, perhaps some will stay.

Mark B.