Mobs don't despawn if they're surrounded within a 20 block radius I think. Since your aquarium probably isn't as big as that, I would use name tags to make sure they don't disappear. I've had this same problem with turtles.
I've had this happen many times too. It hasn't been much of a problem for me recently. Maybe just luck.
Mobs don't despawn if they're surrounded within a 20 block radius I think. Since your aquarium probably isn't as big as that, I would use name tags to make sure they don't disappear. I've had this same problem with turtles.