In the past, internal filters were of the sponge design and not very efficient. The cartridge type which you may have seems much better and similar to a hang on back (outside type). However, I think it's always better to use more than recommended filtration whenever possible. More or bigger filters will keep your tank cleaner and provide more water movement which is beneficial as long as your tank inhabitants agree. More in tank filters will consume more of your interior space, of course. An added external filter may provide a new level of bacterial filtration that the internal filter cannot provide and a different type of water movement to help complement what you already have.
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