Neat game, simple and clean. I understand that it's quick and easy, and do not fault it for its simplicity, but if you ever want to up the difficulty...
Maybe the dynamics can change for higher tier fish?
Some examples:
The keys are switched for more complex fish (i.e. <- appears to the right). Keeps you from becoming complacent.
Use other key presses for increased difficulty! WASD for the small fry, QWERTYASDFGHZXCVB for the middling ones, and the entire alphabet and numbers for the denizens of the deep!
Why not add some undiagnosable diseases (i.e. patients come in with diseases you're not familiar with) and allow the player to give up because they don't have enough information to figure out the disease? It seems strange that patients all come in with a disease that *you* know, only when you know it. But, the learning curve might get too steep, since there are probably a bazillion diseases out there.
 "Fishy Waters"){kind=icon}
 "Medical School"){kind=icon}