Turtles, dolphins, and fish swim at the bottom of a player-made tank. I tried making a 50 by 4 by 20 tank out of quartz in a flat world. 50 blocks long, 4 blocks deep, and 20 blocks wide. I originally filled the tank with a bucket by making a floor and then filling the top of the floor with water, and finally deleting the floor. Any turtle, dolphin, or fish I spawned in would go to the bottom and swim in circles. The dolphins would drown themselves. I then fill the whole area with water using the /fill command. No change. I then filled it with flowing water using the /fill command. All of the mobs swam normally for a few seconds before settling back down on the floor swimming circles. I then spawned into a normal world. All mobs, natural and player spawned swam normal. I tried the same experiment but on a smaller scale. I made myself a small 5 by 4 by 5 tank using glass and sand. 5 blocks long, 4 blocks deep, and 5 blocks wide. The same thing happened, but a few cod spawned naturally in my tank. Those cod swam normally while the other mobs swam in circles. I only used cod, salmon, tropical fish, turtles, and dolphins for my experiments. I didn't try my experiments with elder guardians, guardians, squid, or other land and water mobs.