Sorry, That Was a Strange Thing to Ask - Does the Fish Know It's In Space?

Depends on how massive we're talking about. A big enough aquarium could…pose a whole different array of physics problems for this poor fish.

Hang on, going to go ask xkcd how much water you would need for it to gravitationally bind itself together, and what effects that might have on hypothetical fish.

does i know it's in space?…um…n-no?