Exactly... I think you are confusing what UV lights do. They do not purify the water. They kill free floating parasites. You should not get a UV light until you are sure what flow ratings they require. You have to size them appropriately for your tank. If you pass the water through them too quickly, then you might as well not even have one because they will be totally ineffective at high flow rates.
UV lights are good for PREVENTION... that's it. You won't even know if it's doing anything if everything is working correctly. They are mainly useful for helping kill ICH whenever you add new fish to your tank and it helps keep parasites in check which prevents disease outbreak.
You absolutely need an RO filter and not for any of the reasons people get UV lights. As your water evaporates, you leave behind salts and other dissolved minerals. You want to add the purest water available to top off your tank. Think about it... it is only pure (distilled) water which evaporates. Minerals do not. Therefore you want to add the pure water back to the tank so that your minerals will remain in the proper concentrations. If you add tap water, you add minerals. These minerals do not evaporate ever and so they just build up into higher and higher concentrations as you keep adding more unfiltered water. Eventually the careful chemical balance that is saltwater will fail and you will have problems keeping your calcium levels up while your pH drops suddenly, shocking the tank.
Also, unfiltered water has silicates, phosphates and nitrates (more if it rains heavily). These feed algae like you wouldn't believe. Look on a bag of fertilizer... nitrate and phosphate... You don't want to add this to your tank.
Your most important thing to get is an RO filter. If you quarantine your new fish like you are supposed to then you really don't need a UV light. They are nice to have for maintenance but as for water clarity, you won't notice a difference with or without one. If you REALLY want your water to get clear, then you need ozone, not UV.
