widetrak

I have a gunite pool--18,000 gallons--DE filter--and use regular chemicals. My PH and total alkalinity are in the recommended ranges. I can't seem to keep the chlorine level up at all. If i add the maximum amount of granular chlorine one morning, the next afternoon i have NO available chlorine and only a extremely slight yellowish tint of residue. How is that possible? The water looks great--I am using 3 inch chlorine tabs in a floating dispenser also. What is wrong? tia