You don't need ro water if you are just using fish only and live rock, just keep your lights to a minimum. You need regular instant ocean salt, a hydrometer, redo the media in your filter. You can still use carbon, I use reef plus carbon and a polishing pad similar to fluval. Live rock is needed. It is very confusing at first but once you get the hang of the water changes you will see that it is quite simple to do fish only with live rock, and even simple soft corals and shrooms. Just read, the more the better. Any questions no matter how stupid...ASK! Reef central has some good beginning things also garf.com. Hydrometer is also a must to measure salinity. I mix a half cup of salt into a gallon of distilled walmart water for my water changes in my nano reef. I don't have an ro unit or skimmer, I am running an aquapod.
Id turn that 75 to marine