In almost all cases it is recommended to use rodi water in a reef. There are some people that use tap water with great success but usually they have very low tds water to start with. A ro filter system will remove heavy metals, chlorine, chloramines and other unwanted elements that can wreak havoc on you tank. As far as which system I agree with EriksOasis that BRS filters are great, others to consider would be SpectraPure, AquaFx, Marine Depot ect... I myself found a brand new system on Craigslist and bought the water saver 150gpd upgrade from BRS. The location of your unit kinda depends on how and where your tank is, for me my tank is in the basement. I have my filter in my storage room about 15ft away. For water changes I use a Python to drain and a cheap pump with vinyl tubing to fill it, no more carrying 5gal buckets.