That would be a good option, usually other than chlorine/chloramine, there is nothing wrong with tap water in fish tanks, certainly a lot easier to work with than messing around mixing RO and its associated costs.Maybe I can tee into the mains in my garage where the softener is and use mains untreated for my tank?
As your tank is so small, remineralised RO water would also be another option. Use instructions below to make your own remineralising mix, and ignore whatever your unit remineralises with, extra isn't an issue.
http://www.theplantedtank.co.uk/RO.htm