first all the rodi is just for making new saltwater and top off it is not for the filtration of the tank and is highly recomended if you plan on keeping fish PERIOD..... weather it be fresh or salt water fish. Now as far as tank filtration goes for just a fish only saltwater tank you could just run big power/canister filter to keep it simple, however in all honesty id recomend a sump with bioballs or liverock rubble, and no bio balls arnt bad as long as u keep them from getting too dirty they are fine and do ther job well it when you let them get full of garbage is when you will have a problem. Skimmer allthough recomended not really a nessesity when it comes to a fish only tank, but if you do get one make sure you get a good one as a cheap one is almost like not having one at all. You dont need a dsb for a fish only, for that matter you dont need a dsb for a reef tank. You are gonna wanna get a few powerheads for flow in the tank.