Since you only have a 20 gallon tank you could prolly get buy with a straight DI cartridge type. I had one a few years ago and it puts out the same quality water as an RO/DI unit but the cartriges they use are only good for like 30 gal and cost around 30 bucks after 60bucks for the first unit. Figure this that you can buy RO water or distilled water at Walmart for less than a dollar a gallon and your tank is only 20 gallons 3 bucks a week will do your water changes and top offs and it will cost like 20 bucks to fill the entire tank. For really long term a 130-140 investment will lower your cost per gallon down dramatically. Prolly around 30-40cents a gallon but I haven't changed my filters yet so can't tell you exact.
So basically it's do you mind picking up a few gallons of distilled from walmart? Do you want the ease of just turning on the faucet a few hours a week?