I've only played one game on the hard difficultly level. It was 'hard' so I went back to the easy way! The little fishes were much more delicate in the hard game and had to be treated for illnesses more often, and with larger doses of medication. It took LOTS more time to tend to the little guys.

There may be other differences, too, but that was the one that stood out with me. If you can win a game on the Hard level, you've really learnt the game! ... (don't know why I keep calling it a game, when we all know the little fishies are real)