You can dilute your tap water with RO water. There is no other legitimate way to do it.
You have to take into consideration that a pH drop by CO2 (adding an acid) isn't the same sort of drop that diluting the water with RO would be. Just as some will recommend using peat in the filter, which will drop the pH too, but neither CO2 or peat will alter the KH, which is the measure of buffering and the primary cause of a high pH.
pH isn't quite the important thing that so many fish store clerks and old books say it is, and quite frankly, you probably don't need to worry about it in this sort of situation. Your pH is just indicating that you have a good bit of carbonate buffering (which a KH test would confirm). If you're purchasing your fish locally, see if the store keeps them in similar water as your tap; they'll already be acclimated to the higher KH. There would no doubt be plenty of people saying they keep even the most sensitive acid loving fish in high pH water, it seems to happen every time someone asks about it.
It's better to just not fight the water.
Your GH is acceptable, there really isn't any need to change it, but if you wanted to, products like Seachem Equilibrium are an easier more exact way than using crushed coral, etc.