With a CYA of 50 ppm or so, the chlorine loss from sunlight is usually around half and then there's some small overnight loss. So that would be roughly around 3 ppm of one went from 6 to 3. At 80 ppm CYA, the loss may be more like 25% so going from 8 to 6 so a loss of 2 ppm. The background loss rate (the loss not due to sunlight) is a function of temperature so higher temps have higher losses. In my own pool at 88ºF, I estimate that background loss at around 0.7 ppm FC per day. If you're losing 5 ppm, that seems excessive. What's your CYA level and pool temperature?