A number of things here to explain, most wells cycle about off at 60-65psi and on about 40-45 psi. well water runs 56*F in most areas and depending on how deep your well is. RO membranes are rated at there GPD at 60psi @ 77*F water, so anything less in any combo will give you less performance, also most membranes at there designed rating run 96-98% rejection rate of TDS's except 100GPD they run 90% so stay away from one of those because they will eat up DI resin faster. Well water can be very high TDS or low, if you are getting a water softener run the supply off of softened water. You may need a booster pump do to the low pressure. Jim at TheFilterGuys.biz is great and so are there products, as well as Bulk Reef Supply and Buckeye Field Supply, all you will need is a 4 or 5 stage, more stages is not always better, you will not need a 6 stage, and stay away from Ebay products.
Water is the single largest ingredient in a reef system by far, why not provide the very best? The key is consistency, with a good RO/DI you get the same consistent water quality every time, unlike purchasing water or tap water both of which you have absolutely no control over and may be hit and miss.
Do not get hung up on the word "stages", this is an old ebay ploy to make low quality drinking water systems sound like the best thing since sliced bread. They tack a few cheap filters on like a 10 micron carbon or GAC carbon, both of which are completely exhausted in 300 to 1000 total gallons of water (total gallons means all water passing thru the filter including waste so at the normal 4:1 waste ratio that’s only 60 to 200 good gallons and 240 to 800 waste gallons) or another little dinky horizontal DI with about 6 oz of resin bobbing around in it and call them "stages".
Good vendors don't play this game. Spectrapure, the Filer Guys, Bulk reef supply, and Buckeye field supply, have been building RO/DI systems for the reef hobby for years. They give you a guaranteed systems with exactly what you need, no extra carbons, no small horizontal DI's no extra taste and odor carbons etc.
A single 0.5 micron carbon block is good for up to 20,000 gallons of normally chlorinated or chloraminated tap water, a 1 micron is good for 9000 gallons. No need for extra carbons and the associated pressure drop they cause. A good vertical 20 oz DI like they use is good for much more treated water than two or 3 or more of the small horizontals since it has a true engineered bottom up flow pattern so all water and resin come into contact with each other unlike horizontals that channel and short circuit.
For a reef quality RO/DI 4 stages is perfect, if you want to add something make it a second DI like the MaxCap where you get something for your money.
