For lower pH, unless your kH is sky high (which unless there is something very weird in your water will not be the case) you don't need the RO water, just inject CO2. This will help your plants alot, and won't hurt your fish.
With straight RO water, you will need to add RO right, or the equivalent. Then, you will likely need to add a buffer as well, as you're likely to end up with high pH anyway.
If you mix the RO water and your tap water, you will end up with pH 7.8, even if most of it is RO water.
Bottom line--RO water reconstituted with RO right is expensive. Tap water, if the GH is under 6, will work fine for most plants and fish that like low pH if you inject CO2. If you GH is over 6, consider mixing with RO water to get a lower GH, and inject CO2 to lower the pH.