An External Water Pump is a must have for any marine aquarium. A good water flow is essential for your aquarium tank, especially if you have a reef tank. There is a wide selection of water pumps on the market to choose from, which can be used for your application. External pumps are used outside the sump only. These pumps are installed with a bulkhead from your sump to the pump or with some pipe work. These pumps must not get wet. The advantages of these external pumps are the power and durability which it holds. Each manufacturer makes various size pumps to suit your size tank. A healthy tank with fish only, should have a water turn over rate at about 6-10 times per hour. Many aquarists feel that more is better, especially for a reef tank system. The recommended water turn over rate for a reef system should be at least 10-12 times, but the more the better.