to keep the water in like a bowl or whatever. Might I suggest hitting Costco, Sam's Club or one of those big wharehouse stores. They carry candy, baked goods, desserts etc in the clear containers that are about 3-5 inches deep and about a foot or so in diameter. They are perfect for the "ponds". Mine are quite happy. And they are big enough for a mini filter, rocks, plants etc without being overcrowded. Worked for me. Easier to clean to.