No need to use RO water - tap water will do. Discus can thrive in pH ranging from 5.0 to over 8.0, so long as it remains fairly steady. (i.e. no rapid, very large swings.)
Age your tap water overnite, if need be, to maintain relative
pH stability between tap & tank for wcs.
Here's a listing of advices for the discus novice - follow these 'rules' if you wish to do it 'right'.
D-I-S-C-U-S - 6 CARDINAL RULES FOR NEWBIES TO FOLLOW
First I'd just like to mention once again that discus are hardier than many people think, and are not difficult to keep, so long as one is prepared to accept and adhere to a few key practices that will provide the best chances of success with discus.
This listing is recorded more or less in order of importance:
1) - D - Do your homework well before delving into discus. Read and research all you can beforehand. Googling will certainly help, as well as spending a good deal of time reading the posts and threads on the simplydiscus.com forum, particularly the stickies in their 'Discus Basics'
section, which will provide you with much of the material you need to digest.
2) - I - Investigate and learn of the best sources to get your discus stock. Find those breeders &/or importers that are long time, well-experienced, responsible, reputable, and known to supply high quality, healthy, and well-shaped discus. Buy your discus from one of these sources in order to insure that you get off on the best footing possible.
This is the single, most important factor in succeeding with discus.
3) - S - Set up and plan to follow a strict regular routine of fresh water changes, tank wipe-downs and cleansing, vacuuming of wastes, and regular filter and media cleaning, changes, replacements, and maintenance. Be fully prepared for the kind of commitment it takes to produce and maintain the highest water quality and conditions that you can.
4) - C - Carefully consider the type of tank set up you start with. Make sure the tank size is ample enough to start with 5 or 6 discus. Don't be tempted to begin with a tank of less than 55 or 60 gallons, and don't try to justify going smaller by just getting 1, 2, 3, or 4 discus for cost or other reasons.
Wait till you have sufficient resources to get a proper-sized tank, and the suitable size and number of fish to insure continuing good health and harmonious discus sociability.
Do not start with small, undersized, very juvenile fish which have not yet developed a more mature immune system, are more demanding to raise properly, and much more prone to health problems and other issues. Get fish of at least 3.0" in size, preferably larger.
5) - U - Undertake to start off with a bare bottom tank, unless you're getting fully adult fish and have previous good experience with maintaining a planted tank. If you must have some decor, limit yourself to a very thin sand substrate layer, and perhaps a piece of driftwood with just a couple of small plants attached, or one or two potted plants.
Once you gain several months' of experience getting to know your discus' traits & behavior, and your discus get larger, then you may proceed to an aquascaped environment. Feed a varied diet, several times a day, and learn which foods will achieve a nutritious diet, by researching.
6) - S - Simplify. Keep things as simple as you can to start. Don't complicate your start with discus, at least at first, by placing them in a heavily planted environment, using CO2 and a strict fertilization regime. Make sure your tank is fully cycled before adding the fish, and don't be tempted to alter or change the pH of your water, or modify your water conditions and params by using chemicals of any kind. No need to use RO water or adopt any other procedures that would tend to complicate what should be a simple start to your discus launch.
Follow these 'rules', and there's little doubt you will succeed with discus !
May I also suggest you have a read of my "Beginner's Guide to Getting Started with Discus", located here in the Fish section - click on the first & only Sticky for the Article.