If you used trichlor last season, and had zero CYA on opening this year, you might be experiencing the ammonia demand that we're seeing a lot of this year--happens sometimes when the CYA breaks down over the winter. Try testing for chlorine at night, and then test again in the morning before the sun hits the pool. If you have more than a 1 ppm loss in that time, then you have something in the water that the chlorine is battling, and you need to raise it to shock level and hold it there until whatever is there is gone. If it's ammonia, it can take LOTS of chlorine, but it needs to be done.