The Iwaki Motor Drive Aquarium Pumps come in versions with a low max head for circulation and high pressure versions for canister filters & other applications where high head pressure is needed. RLT type, handle back pressure better and can be used for almost any application. Especially recommended for high-head situations or when using cartridge (pressure) filters. RLXT type, are best suited for circulation only - that is, where water encounters very little back pressure--for example, returning water from a sump. Excellent where a high turn-over rate is desired. WMD, have American Made Motors. MD, have Japanese Made Motors. Related Products: Iwaki MD & WMD pump replacement parts.