I ordered my tf-100 test kit & I should soon receive it. In the meantime I purchased a basic tester. There was some alge growth. So I put a 1lb bag of shock and ran the filter all night. And I scrubbed the pool. Pool is clear & chlorine is at good level & ph is good. Parents went swimming yesterday. (I didn't) &'they said they itched real bad. What could it be? Please help. Btw... I printed pool school & read it over and over. Thanks.