^Right, they can't survive long underwater. If you want aquatic animals that can be bred underwater for food, you might consider snails or shrimp. Just be sure that the shrimp won't make a meal of the fish, or the snails a meal of any live plants. Another solution perhaps is to keep a second, smaller tank, e.g., a 20 gallon, and then raise shrimp / snails there, then cull some each week and toss them into the fish tank. Ready made, live food source that you can dispense as needed.