I have a problem. I built an aquarium that has an Axolotl tank that is separate from my Tropical fish tank so that way they don't kill all the other fish. I thought it was a cool "Finding Dory" inspired project, but I realized that they will still go after fish even if they're not in the same body of water. They're stuck at the top of the tank trying to go after them. You can see them in the top right. My friend had an issue like this as well. He made an underground pond that was near a surfaced-river. All his Axolotls swam to the very end and tried to go up to kill the cod he had in the river. (He built dams to trap them for decoration near his base)

Here is a picture of the two tanks. Please patch this: