What kind of "internal filter"? For saltwater, you get most of your filtration from bacteria on rock and sand (biological) and a skimmer (mechanical). Actual filters are not really necessary. You do want enough current to keep stuff (Poop, food, etc ) suspended so that the skimmer can get it. There are small power heads out there that don't push very much water . When there are dead spots , that stuff piles up and causes nitrates. Is this LFS
someplace like PetCo or a privately owned shop? Them saying no power heads are necessary is very questionable, IMO
, and I would be very careful with future advice from them. Ask here and your advice will probably be better.