i feel like 99% the time you see a fishtank in a movie you know somethings happening with that fishtank

To be fair, fish tanks are heavy and hard to move. There's no way set decorators are going to go through the trouble of having one in scene. (Not to even mention how they're reflective/often lit, so the gaffers wouldn't be a fan either.)

Chekov's fishtank

Checkov’s fishtank