Catfish crash

This is my second experiment with catfish in my pond.

Two years ago I put in 25 and a pound of minnows and within a few weeks
they started appearing at the surface cruising around and shortly
later, floating dead. By the end of summer, they were all gone.

This year I only put in 12 and the minnows but it looks like deja vu
all over again. I didn't keep count but I just fished out another one
today and this is close to if not the last one.

I installed an aerator a few days ago but two have died since.

There are several mitigating circumstances.

1. Because of my experience, the hatchery lady threw in 6 albinos for
my trouble. I have not found any dead albinos... just the regular
channel cats.

2. The minnows are so happy they are breading and there are zillions
of little ones all over the pond but many head in towards the bubbler.
I suspect this is just standard behavior of swimming up stream but I
suppose it could indicate low oxygen elsewhere but none of the adults,
cats or goldfish are there.

Any thoughts?

My pond is 15 years old, natural bottom, 40 ft diameter, 6 ft max
depth.

js

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