External Aquarium Water Pumps consist of an external motor which sucks water from the aquarium or sump and pushes it out the outlet. This is the most common type for sophisticated freshwater, marine and reef aquariums. They are more powerful than submersible pumps and help keep the heat out of the water. Commonly used as return pumps to feed filter water back to the aquarium and to drive aquarium equipment such as canister filters, protein skimmers and uv sterilizers.