I swear people live in evap free environments, heh. 3 gallons a week in top off for my 20 give or a take a few cups. That's not including water changes which at 10% (on the low end) is 2 more gallons. Obviously a 10 won't use as much but it'll still be 3 gallons a week. You'd be able to recoup the cost of an RO unit in gas/hassle/distilled water very, very quickly. I'd explore all avenues in getting your own RO/DI before resorting to Distilled. If you can access a LFS and get a few of those 5 gallon blue water jugs from a hardware store and store them someplace that might be an option as well. I did that with one my early saltwater tanks, it wasn't terrible.