As far as a RO/DI filter if you want really clean water, a Four stage will do fine RO, Cotton, Carbon, DI filters. These will bring your water down to 0.000 TDS (TOTAL DISSOLVED SOLIDS) you pretty much can't get much better its basically pharmaceutical grade water. A five stage wether is has 2 DI (de-ionizing) stages or a expensive uv sterilizer in some opinions in the hobby is redundant. The extra de-ionizing phase will help extend the life of the DI filters because you rotate the old with the new, but unless you have a very large setup, it may not be much savings in the long run. A far as having a uv sterilizer , many consider it to kill off beneficial microbial, which if so will make the tank less productive in health and growth development for corals and algae . Unfortunately I don't know enough information about UV sterilizers to tell you the benefits. Generally if you don't want to spend a bunch of money wether it be on the unit alone or the filter changes involved with a 5 or 6 stage unit, The four stage unit is plenty suitable for most reefs!!! I Hope this helps thank you for reading!!!