I'd love to see the cichlids in the game swim more like cichlids irl, using the pectorals to move forwards and backwards in the water and using the body and tail mostly for darting to escape or catch prey.
It would also be very nice to have the gobies and blennies "land and perch" on decorations and the sand in the tank.
I'd also like to see some tube worms like the featherdusters.