In a freshwater tank you would use one if you wanted more current, like to create an endless river setup. In a saltwater tank, they are used to move the water through live rock, which acts at the biological filter.
Most powerheads are also set up so that you can stick an airline tube in a small hole and leave the other end free in the air, and the vaccuum action of the water pushing by it will suck air into the water stream and help to aerate the tank that way. Usually this is called a venturi arrangement. I set one up in my 75 gallon turtle tank like this to get some more water movement to keep the waste kicked up where the filter would get it and to add some extra aeration.
The most important things for them is the type of fish and to create circulation. In a planted tank that size they are almost a needed item. If you have slow water fish and few plants then they powerheads may not be a good addition. Lots of uses, what do you plant to do with the tank and what type of fish will help to decide if they should be needed or not.
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