R/O water is fine to use for fish, you may need to add the mineral/electrolyte back for fish/plant health, SeaChem has a good one called "equilibrium" that doesn't cost much...
Have you ever tried to use your water and do you have the level for the iron an sulfur...you may just need to add carbon and filter the water yourself, just change the carbon out every 2 weeks.
You can keep water filtered all the time just for the fishes weekly water changes too with an extra container with a filter and carbon..