NB, hi and welcome to the forum. I guess I'm the first one to see your post so I'll try to give you the answer. It sounds like you are using OTO to test your chlorine, turns yellow in cl's presence. It's my understanding that the immediate reading is for free chlorine, the stuff that does the killing, and any subsequent deepening of the yellow indicates the presence of combined chlorine, the stuff that has already done it's killing and is now just useless and smelly. A number of people have questioned whether or not OTO actually works for this. FAS or DPD tests can test for both free and combined chlorine which is useful when trying to combat an algae bloom. In short, the kit you have has it's uses, but having a way to accurately measure combined chlorine is a good idea so that you know you have sufficient sanitizer in the pool. Hopefully this is at least somewhat helpful, if you've any further ?s, please feel free to ask, the nice folks here are always willing to help with just about any pool problem.