pH of RO Water
First, when you say the pH is low, how low? i.e. what is the actual pH.
The pH of RO water will always be below 7, since water naturally absorbs CO2 which converts to carbonic acid in water. This is nothing to worry about, as it has very little buffering power. I have a 120 g tank with an automatic RO/DI water topoff, and have a consistent pH reading of 8.2 - 8.4.
You probably need to add some buffering capacity while raising the pH. Try adding something like SeaChem Reef Buffer and get the alkalinity up to 7 and see what happens.