[Klaus]

Why not a submersible DJI Osmo Action 4 (1" sensor) with a remote control?
Not sure whether it supports photos, but maybe worth to investigate?

As far as conventional cameras are concerned - interesting question which one would do fish detection AF ;-)
Sony has still the best AF.

So a7 iv or something else ? Under water might be a cool twist but generally not effective for what i do as the tanks are heavily planted and usually i'm trying to capture more scenery. The blackwater aquarium is more of an exception though it has some nice hardscape once the tannis thins a bit.

Everything is a money question in this game ...

I'd probably go for the A7C II for the latest in AI AF if you want to optimize it and don't mind the low-res viewfinder. Or an A6700.
As far as fast lenses go ... well ... there are a bunch but zoom lenses won't go faster than f/2.8 normally.
I suppose that due to the aquarium glass, you won't need the greatest lenses around so a Tammy/Sigma would do.