I used to when all I had was a 14g nano and only needed like 2 gallons a week, it was worth it at the time since it cost barely more than a gallon of RO from the grocery and I didn't have to mix myself. Now with 3 tanks to take care of, it's totally paying itself off that I have a RO/DI unit and big buckets for mixing my own water every week. I think I figured it out that 5 months of water changes pays off the unit compared to buying water when I need it.