The pH in the reactor depends on the type of media you are using. Typically though the pH in the reactor is between 6-6.5. Some media brands, such as A.R.M., the pH doesn't have to be as low to dissolve the media, say 6.8.
To lower your effluent pH from your reactor, you can do two things
-decrease your effluent drips per minute, and/or
-increase your bubbles per minute of CO2