you don't need RO water for Discus.
but if you want it to bring down for breeding, i recently found home master has a hydroponics unit that is priced okay and makes a lot of water. i bought one of their units for drinking water, and i also use if or fish. they were good with customer service when i had an issue with the valve.
https://www.theperfectwater.com/Gard...ponic-Filters/