I know what you're thinking, No Corals then why do I have to have RO water. Well, you have Live Rock in there. If you want to keep it live and cut down on nuisance Algea then it's nessary to have better water. People say I Use Tap water and I'm fine but really what does your tank really look like? The only aspect that changes for a FOWLR is your lighting. You can use a normal light no problem, T-5s are better to bring out fish colours but you could use a Home Depot T-8 for 40 bucks. But in the end it really comes down to you. If you want a Sub par system then fine but if you want the nicest FOWLR then the possibilities are there.