What size do you need, how many gallons of ro water do you need. Generally you want a unit from a hydroshop, that is designed for continuos use. Most of them available at the big box stores are not. I spent about 1/2 that amount for my Spectra Pure System, replacement filter are around $26.00, last about 3 months for my four table flood and drain op. You will want to have a sewer line, the ro systems reject water at a 5-1 ratio, so 100 gallons or ro means you will 'waste' 500 gallons. VV