Well I think I finally figured it out. Bacterial. Cause was an
overly dirty canister filter. My real tip off was when I removed the
top to fish out a dead angel. It stunk - not like a normal aquarium
but rotten type smell.
I fairly recently (6 months) switched from penguin power filters to an
Eheim Pro II 2026 canister. No cleaning in about 6 months. Water
flow seemed still strong. UPon opening up and cleaning the canister,
it was very dirty. Dirt on the "clean water side" (e.g. top)of the
filter. So I cleaned out the filter, careful not to disturb the
noodles and other filter media. Just rinsed in tank water. I did a
thorough cleaning of the top , originally white filter media, using
tap water. Also added back one of my penguin filters with a new
filter media w/ charcoal. That was all yesterday. Today the large
angel is eating. Yahoo!!

I was really stumped by this. But the cause was so obvious.
Exacerbated by using a new kind of filter. I mean I have kept fish
for 40 years and still this stumped me.

Thanks to all who responsed with suggestions.

Bob