Submersible aquarium pumps have the advantage of being very quiet. They are usually used for high-volume pumping in large aquariums or ponds. They can also provide some heat to the tank because the heat generated by the electricity is fully contained inside the aquarium.
Choose the Right Size
Pumps are rated in gallons per hour. Depending on your filtration system, you want to "turn over" the water about three times per hour for an external filter or wet/dry filter system, and about five times per hour for a contained filter like a canister. A power head is a small submersible pump that is usually used on top of a downtube for an undergravel filter.
Streamline the Plumbing
To make your pump as efficient as possible, avoid right-angle bends in the tubing, both leading from the pump and leading to the pump. Every bend you put in the lines will cause back pressure and make the pump less efficient. Re-test your pump after it is connected to all the plumbing to see how long it takes to fill a gallon container. Divide 3,600 (the number of seconds in an hour) by the number of seconds it takes to fill the container. That will give you the actual gallons per hour.
Install the Pump
Follow manufacturer's instructions for installing the pump. Install the pump in an area where it will not disrupt the aesthetics of the tank. If it is sitting on the bottom of the aquarium, make sure it is flat and level and fittings are not strained or twisted. Smaller pumps can be attached using suction cups supplied with the pump. Important: Make sure you leave a "drip loop" in the electrical cord before it enters a live outlet. This is a U-shaped loop of line that allows creeping moisture to drip off the line before it reaches the outlet. This will prevent corrosion and shorting of the outlet.
Check the Setup
After first installing the submersible pump, you may have to adjust the heaters to keep your tank at the right temperature. Check the temperature of the aquarium water every hour or so to make sure the temperature is not exceeding a safe level for your fish. A large pump in a medium-sized tank can easily raise the temperature of the tank to over 90 degrees--make sure your fish can tolerate that temperature before you install the pump.