Myself, for a 5ft aquarium I would use either one 6100 or two 6000, the 2 6000 of course would be better because the flow could come from one side than the other. The 6000's flow carries about 4ft and the 6100 about 7ft. I generally try to stick with the 6000 in anything less than 6ft long. The big question is what do you intend to keep and what other sources of flow will you have?