The art and music on this are utterly phenomenal, and I'm shocked you were able to pull off something like this in Ren'py. If the game gets expanded, I can imagine sinking hours and hours into this one... aquatic pun intended?
One thing that I noticed though was the bottom of the tank was slightly annoying- if a gem spawned too close to the bottom, it would instantly vanish, and occasionally fish would just swim away from the food, leaving me to just wait for it to slowly despawn before I tried to feed them again. That being said- again, it's astounding to me you got something like this working in Ren'py lol. Amazing game!