Most come with the extra faucet, so you don't get stuck using
RO/
DI water to wash your hands, wash dishes, etc.
There are different 'stages' you can get. a 2-unit piece may only be a carbon filter and an
ro membrane. or it may combine the membrane around the carbon filter, then have a secondary
DI phase. or a 3-piece would have all 3 split out and be the most filtering you can get.
Then you have output...10gallons per day? 50? The higher the value the higher the price.
is this for a
SW tank or
FW? Unless you have awful water,
FW should not need
RO/
DI. And before
RO/
DI, many, many many people kept
SW tanks without any problems. Point is, if nothing appears broken, don't start fixing...