I made a giantic marine life preserve thing in Minecraft and just let fish spawn as they wanted, and I have noticed that with large groups of tropical fish they will sometimes push eachother around making the pushed fish fly over the the water without being able to move and getting stuck, sometimes the same thing happens exept its because the fish has been pushed up in a corner. This happens mostly with the "leader" of the group which then makes the rest of the group stuck too. Its quite annoying even tho it can be fixed by picking up the fish in a bucket of water and placing it back into the water, but if you have more than 20 groups of fish like me it easily ends up getting tiring having to constantly move around the fish.
How to (maybe) reproduce:
Make a big aquarium for tropical fish. Place a big group of tropical fish (around 6ish). Wait for the fish to star pushing eachother around.
I added some pictures so you can see my problem. Some of the pictures is to show that even after pushing the flying fish around, it does not go back to normal.
(Also, I hope my nightvision particles arent disturbing, its just so the water looks clearer)