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Would something like a Fluval 104 be too much for a 15g tank? Since the Fluval has more media than any HOB filter, wouldn't this be better has you can house more bacteria?
A 15 gal tank will not house enough fish to make all that media necessary. Keep in mind that bacteria will only grow if there is food and room. You will offer plently of room, but your fish will not supply enough food to have the bacterial colonies living in all the media.