The acceleration acts only on the water not on the fish so as long as the fish
Rubbish.
Ok – explain this please.
Maybe I’m missing something but why look at the effects of weightlessness on fish when they inhabit a zero gravity habitat on earth?
Floating is not the same as zero gravity. Take a hot air balloon ride, then pour a glass of champagne, see which way the drink falls.
The water around the fish supports it, but gravity still affects it.
Not sure what your point is? Of course the chanpagne would fall into the glass gravity still acts on the champagne as air is not a dense medium. The effect of gravity on water pressure is minimal compared to water depth. You cannot compress water so along as the tank that holds the water that holds the fish accelerates at the same speed as the rocket the effect on the fish is tiny. If the fish were forced to the bottom of the tank there is no way they would survive the journey into space let alone swim around to have experiments performed on them.