Given it's only a 10 gallon tank you could also just buy the RO
/Distilled water by the one/five gallon from the lfs
(if they have it) or grocery store. Or even pre-mix saltwater from the lfs
Long term buying the unit as ccCapt suggests is the better route.
Live stock-wise, do some research on snails, crabs, fish that eat algae and they types of algae