I opened my 4500 gal above ground pool and added a gallon of 12% liquid chlorine. After circulating a while, the test strips show no free chlorine. So, i added a second gallon. Same result. I've never seen this. Usually one gallon is fine to start the summer. Any ideas? is the chlorine bad or is it being immediately used up? We had a bad rain storm the night before and there was a lot of organic material in the pool from overhead trees. I cleaned it out before adding the chlorine.