Oh man, I didn't realize that was what the game meant when the 20 points gallery hint kept alternating between 2 images. Thank you so much!

Did a black and purple texture appear above the shop button? If you have visited all 5 entrypoints in a run, it should be there. Click it and you'll get the secret ending.

Sometimes, when you punch the fish, a 4 digit number will come out. Type that number on your keyboard. It's a different number every run. I honestly don't understand the Petscope one.