Depends on the size tank you're setting up, and if RO
water is available at your local grocery store. I can get RO
for 12 cents a gallon, and due to awkward plumbing in my house, its just easier to pickup what I need at the store.
A decent unit will run $100, and the refill cartridges run $50-$75 from what I've seen, but buying a 4pack seems to be cheaper.
It also depends on the quality of your tap water. People around here get by with tap water, as we have no readable nitrates or phosphates in the city tap water, and a fairly high alkalinity, which saves a little on alk
. buffering products.
For a FO
isn't necessary, but once you go to reef lighting w/ corals, that's when you gotta make a decision on getting RO
water or installing your own home unit.