Have you tried improving the water circulation in your tank, to make sure the CO2 enriched water reaches all of the areas of the tank? That is necessary to avoid dead water areas that get depleted of CO2. Usually we set up our tank with good circulation, but the plants all grow, filling up the volume of the tank and stopping circulation in many areas. Then we need to do a major pruniing job, readjust our water circulation system - powerheads, spray bars, etc - and make sure the drop checker is in a typical area of the tank, not an area with unusually good circulation. The thing that works best to stop algae from starting to grow is good healthy plant growth, but once algae gets started CO2 alone isn't likely to kill off that algae.