RO Units


Can anyone give me an insight as to what kind should I buy ? I have a 30 gal tank with
40 lb live rock,three inch thick substrate and an inch of live sand on top.
A seaclone skimmer, a 8 watt UV sterilizer, fluidized bed filter and mechanical filter with bio wheel (all hang behind the tank. I have a small yellow tang, a small blue tail damsel, a cleaner wrasse, three hermit crabs and a cleaner shrimp.
I use R/O water for water changes every week.
And I also have four corals in the tank which have become more shy lately and I am assuming because of nitrates
My Nitrates have kind of gone up to 10 and don't come down even after a water change. I do a 20% water change max every week. I know that a R/O unit will help but
1. What capacity should I go for ?
2. Are these wall mountable ?
3. Do I really need one ?
I was looking at a Kent Marin Maxima 50 gal /per day unit. Does anyone have this unit installed ?