Ray-JrMALL Aquarium Fish Tank Biochemical Sponge Filter Air Pump They are made from durable plastic, and suitable in salt and fresh water. Designed with a weighted base, so they can stand firmly in the tank. They are absolutely safe for fish and best of all, they won't die, rot or cause any algae problems. Their color may not appear as exactly as in real life due to variations between the computer monitors and nacked eye colour difference.