OASE FILTOSMART EXTERNAL FILTERExternal Aquarium Filter For a Simplified Filtration Compact filter with large filter surface Model 100 offers 2 stage filtration: biological and mechanical filtration; Models 200 300 offer 3 stage filtration: biological, mechanical, and chemical filtration. Multi staged filtration provides clear, healthy water and a high surface area for beneficial bacteria to colonize. No mess maintenance Locking features are integrated into the inlet outlet