Join Date: Jul 2010
Location: Outside Philadelphia, PA
Something that should be pointed out is that RO water has very little mineral content, ie no buffers. This means that it's not going to effect the pH of your tank very much. This is why you tank stays a a pH of 8.2 or so. This is also why you get a pH reading of 6.4 from your RO water. It's really not a problem. Aquarium grade test kits are not that accurate.
Test the GH and KH of your water and let us know. With the RO water changes you have been doing these numbers might be extremely low, and you may need to adjust those before you attempt to alter pH.
Also, most fish and plants can adapt to a wide range of water conditions, so I don't think you have anything to worry about unless you plan on something very unusual.