Reduce your pH to 7.2 (or just a little lower). This will free the chlorine to do its work. Adding chlorine to water with a pH of 8 or higher is a waste of time and chemicals...the chlorine will not work. Use a simple test kit (not the OTO) to test for free-chlorine, and combined chlorine levels; pH and acid demand, then total alkalinity. Follow the directions in the test kit for adding acid (probably necessary).
The basic-4 test kit is about 14 dollars at Home Depot. It will save you a lot of headache and money in wasted chemicals, as it takes the guesswork out of what you really need. If the water chemistry is right, and your filter is clean, your problem will go away.
