unit is used to make water for water changes and top offs. You can get an adapter for it so it just screws on to a faucet and you just hook it up and use it when you need some water.
The size doesn't really matter except something like a 100 gal
/day unit has a bigger R/O membrane and it will last longer than say a 50 gal
I got mine on Ebay from FilterDirect.