A Reverse Osmosis/ Deionization (RO/DI) unit is used to purify water, prior to its introduction into a reef tank. RO/DI water can be used to mix with salt and can also be used as top-off water. This is the purest form of water available to the hobbiest and is needed to prevent outbreaks of algae. Regular tap water has chemicals and impurities that will affect the tank adversely. In general, the RO portion of the system will remove 95% of the impurities in the water and the DI will remove the rest. Since I have low tds to begin with, all I need is a efficient RO unit. BTW, tds refers to ‘total disolved solids’ within water and is a good measure of how ‘clean’ your water is.
I decided on the Spectrapure MaxPure MPRO-300 RO unit. It produces 300gpd of water per day(provided your incoming water is 77 degrees and water pressure is 60psi). Since my house is on well water, both my water temperature (55 degrees) and water pressure are low(40-50psi). I was only getting about 24gpd with my old 75gpd rated RO/DI unit. After a short test, it looks like this unit will produce over 120gpd for me. Not bad at all! Pictures below.

Sediment filter, carbon prefilter, carbon filter, and 300gpd membrane. A nice added feature was the built-in pressure gauge.

Size matters! Compare the size of my old unit’s membrane (filmtec 75 gpd) and the Spectrapure’s 300 gpd unit.

Here’s the unit mounted on the wall of my fish room. I added a tds meter and it reads 0 tds!
Special thanks to Charles over at Spectrapure for answering all of my question! Here’s some links with more information:
http://www.spectrapure.com/
http://www.premiumaquatics.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=SP-MPRO-300&Category_Code=spro