Rite-Size B fits Penguin 150B, 110B, and 125B. Rite Size C fits Penguin 200B, 350B, 170B, and 330B. Replace cartridges every 4-6 weeks as needed. The Rite-Size filter cartridges contain faster-acting Black Diamond premium activated carbon for maximum mechanical and chemical filtration efficiency. Heavy-duty plastic frame with floss and carbon traps dirt effectively. Each pre-assembled aquarium filter cartridge slides easily into place, providing superior mechanical and chemical filtration, and making replacement fast, dry and effortless.