My local aquarium which happens to specialize in reef tanks sells RODI water for $1 per gallon. I have a 20-gallon tank which only needed 16-gallons of water to fill after I had the rock and sand in there. So a 15-20% water change (I am going to do weekly changes thanks to all of the great info provided by other reefers on this site) of a 20-gallon tank is 3-4 gallons. I have two 5-gallon containers with lids and I go once every 2 weeks to fill them up for water changes and top offs. The back and forth to the store, plus the actual water change when I get home takes about 90mins total, and again, I only have to go once every two weeks. Doesn't bother me at all.