I am assuming that by "drip" you mean the bubble rate? If you're effluent is dripping out too fast, that might be the problem; the water may not be in the reaction chamber long enough for the CO2 to lower the water's PH. Also, by what means are you measuring the "coral not melting fast enough" (I'm assuming, again, that you're talking about the crushed coral skeleton that makes up the reactor media). There should be a PH meter involved here somewhere, specifically directly where the effluent leaves the reactor (have the effluent drip into a glass that spills over into your sump). And have you been monitoring your calcium levels to see the results?