FRESHWATER MUSSEL. Two or three of these to every 10 gallons of water in an aquarium can do a great deal of good, for they live by extracting algae and other bits of living matter from the water around them. In an outdoor pool their use is questionable, for they die easily, and as they decompose they foul the water unless quickly consumed by snails. Dead mussels can quickly be spotted in an aquarium and removed, but in a pool, where several dozen of them would be required to do any good, they can easily decompose unnoticed. Even in an aquarium they are subject to some criticism, for they move ponderously, roiling the water and often uprooting aquatic plants.

   FROG. I enjoy the sound of a frog in a pool, although many water gardeners consider them dangerous to fish. It is true that a frog can eat a fish almost as large as itself, but this possible danger is not necessarily serious. First the frog has to catch the fish, and that is a difficult feat unless the fish is one of the extremely slow-moving fancy breeds. We have thousands of frogs living in our commercial ponds, and don't feel that we lose any appreciable amount of livestock to them. Frogs are worthless as scavengers, though often sold as such.

   SALAMANDER. Another pool pet frequently cataloged as a scavenger, which it is not. It lives principally on insects and insect eggs. Its quick, darting movements around the border of a pool add a certain interest, and it does no harm.

   TADPOLE. As aquarium scavengers, tadpoles have doubtful value. It doesn't take them long to learn to eat food thrown to the fish, and then they are scavengers no longer. They are active and keep the sand bottom in constant turmoil. If they amuse you, have them, for they do no real harm.

   WEATHERFISH. An interesting aquarium scavenger and a very good one, which inhabits the bottom of the aquarium, frequently burrowing into the sand as it feeds on matter that settles there. This constant burrowing makes it valuable in larger aquariums, where the sand might otherwise pack down too tight for plant roots to penetrate it easily. Although they grow several inches long, a 3- to 4-inch size is best, for they roil the water less and do not uproot the aquatics. They do their good work in pools, too, but, being bottom dwellers, cannot be seen.