This has been a problem in many of the versions and I've been hoping it would get corrected each update, but it never is. The issue is that fish put in artificial ponds and tanks, tend to all keep to the very bottom of the enclosure and spin in place endlessly, instead of swimming around the entirety. Some fish will keep to the sides of a tank as though they are stuck. This issue happens with all types of fish, even cod and salmon. The cod and salmon will occasionally swim normally for a few seconds to a few mins after the player spawns into the game but they soon keep to the bottom and spin in place like all the rest of the fish. It is very unappealing. Strangely enough all types of fish will swim just fine in naturally generated rivers (although sometimes they only keep to the lowest block to swim where they get clogged up together.)