Activated carbon insert for Zoo Med's 501 External filter. Zoo Med's activated carbon filter media is made from the highest quality carbon available for superior absorption of ammonia waste products produced by captive turtles, amphibians, or aquarium fish. High ammonia levels can be deadly to amphibians and aquarium fish and will cause eye problems in captive turtles. Activated carbon is one step in reducing ammonia in your terrarium or aquarium.