High chlorine loss overnight indicates some sort of 'goo' in the water that chlorine will burn up. What sort of filter do you have? Some times the 'goo' is in the filter. If you have a cartridge or DE filter, it might be a good time to break it down and clean it completely.

BUT . . . .keep adding 5 gallons of plain bleach each evening, and then test in the morning before the sun really is on the pool.