That's simply not true. The active chlorine level is roughly proportional to the FC/CYA ratio. There is no such thing as a "maximum effectiveness" for CYA at 70 ppm. CYA continues to protect chlorine more and more from sunlight at higher CYA levels, though obviously there are diminishing returns for that. But as for CYA's affecting chlorine effectiveness (i.e. the active chlorine level) that continues to make chlorine less effective as the CYA level climbs. There is some effect of chlorine bound to CYA so at some point that starts to dominate, but the chlorine bound to CYA is around 150-200 times less effective than unbound chlorine so for normal CYA levels (i.e. 80 ppm and below) we ignore that effect.march2012 said:
If your FC is 35 ppm with a CYA of 200 ppm, then that's certainly enough to prevent algae growth (7.5% would be 15 ppm) so the cloudiness is likely just having a relatively high bather load and it just takes time for that to clear up. Faster turnovers would probably help if the cloudiness was due to insoluble chemicals such as sunscreen. I would guess that at a CYA of 200 ppm that an FC of 7.5 ppm might be enough to prevent algae growth rather than the higher 15 ppm. However, we are now entering the area of very high CYA where we do not have enough reported experiences to validate theory.