Cary,

The key is whether you are using more chlorine at the lower CYA levels with correspondingly adjusted FC levels according to Ben's Best Guess CYA chart. If you were maintaining a minimum of 5 ppm FC before (so obviously higher after you add chlorine so that it ends up at this level), then at the lower CYA level the FC would only be 3 ppm instead. In reality, the minimum chlorine amounts are more like the 0.03 column in this chart and the ratio of CYA to FC is what should remain constant.

So unless you maintained 3 times the FC level at a CYA of 90 than you did at 30, then the comparison isn't a fair one. We are looking into additional CYA benefits in a technical thread so any more detailed information you can provide regarding the FC level you were maintaining before at 70-100 ppm CYA and what you have tried to maintain now at 30-50 ppm CYA would be helpful to know. There may very well be a benefit to the higher CYA level beyond what has been traditionally said regarding diminishing returns.

Richard