Then adjust the pH within the reactor to approximately pH 6.5 to 6.7 for dissolving the medium. First, measure the pH of the effluent exiting the reactor with a test kit or pH probe (I recommend a pH meter as most pH test kits are not sufficiently accurate). If the pH is too high, reduce the effluent flow rate; if the pH is too low, increase it. Allow a few hours for the reactor to respond to the changes, and repeat this step until the pH value is between 6.5 and 6.7.