What's the most amount of fish you've ever seen in one place?

last night in my dream i saw a huge harvest being made from a swimming pool

The aquarium in NOLA

Thousands of salmon running up rivers several times.

Scuba diving in the Red Sea.

There's a lake in northern iowa that used to have swarms of bullhead (mini catfish), until a fish appeared that was considered invasive in the early 90's. They poisoned the lake and all the fish died, it has never recovered.