I have experimented with making tanks at various depths and it always seems to be the same thing. Aquatic animals either simply swim in circles at the bottom or they get stuck, which completely ruins making a nice fish tank. I've looked and have seen that this seems to be an ongoing issue for quite some time now.
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[Mod] Umija5895M
Note also that mobs cannot swim "up" in downward-flowing water. So you need to make sure all water is "source" blocks or mobs will be stuck on the bottom. If you only put water source blocks at the top of your tank, the layers below the surface will be downward-flowing and push mobs to the bottom.
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This does not appear to correct the issue. I completely emptied one big aquarium and one pond and filled by bucket one square at a time. Fish are still getting stuck. To make sure it wasn't something I was doing, I went to generated lake area and released sea turtles into the water and for the past two hours, they have been spinning in circles.