This site only contains information on freshwater fish, for now anyhow * The best size aquarium for the beginner aquarist is a 10 gallon tank. I like fish. Some fish can be gentle. Others are aggressive and semi-aggressive. I would try to avoid those type fish. A fish tank can be kept on a desk, counter, table, window seal...as for me, on top of my dresser. A good size fish to start out with would probably be an inch or two in size; such as a Molly, a Platy, or a Neon Tetra. In my family, a traditional fish to buy for every tank is a type of catfish known as a Plecostomus. The Plecostomus likes to feed off of slime that grows on the sides of aquariums. A Plecostomus can grow anywhere from an inch in length to over one foot or so. The Plecostomus will grow to the size of the aquarium that they are raised in (if purchased at a young age).