In order to keep it away, you have to kill it all off--not just the stuff you can see, but also the stuff you can't.....and the only way to do that is to sustain your high chlorine levels until it's all dead. You'll know when you've reached that point when you can go from sundown one night to sunup the next morning without losing any chlorine--but you need a good test kit to be able to accurately measure that. If yu'll look at the K-1515 in the testkit link in my sig, that is the chlorine part of the K-2006. If you want to go ahead and get that, it would at least tell you accurately how high your chlorine is....