Your nitrite level has nothing to do with water quality. Going to an R/O setup will not fix the problem. Nitrite is only present when the tank is not fully cycled. I offer cycled sponge filters to hobbyists in my area for free. Maybe there's something in your local fish club that would offer you one. If you're getting nitrite readings in your tap water, I recommend taking a sample to a fish store for reanalysis. I suspect these are not accurate readings.
Frankly, many people starting out get caught up in water chemistry. You say you have 4-5" discus doing well for 2 months in the tank. Why are you trying to change things? Don't worry about water chemistry. Just keep up with water changes. Nothing comes close to having clean water for producing big, healthy discus.
Good luck, Willie