Janet, you absolutely rock, thank you so much for your quick and thoughtful responses.

For daily chlorination I use Tri-chlor pucks; cal-hypo for shock. It's interesting you mentioned the dramatic drop in CYA, last year everyone told me the problem was high CYA (and at 120 ppm, I would agree), but when I tested first thing in the spring, the level was almost nonexistent. I think you're theory about its bio-degradation into ammonia over the winter may be spot-on. I think I will continue shocking regularly until chlorine holds and water clears, then I'll add the conditioner.

I mentioned that I've had perennial problems with this pool, which would make perfect sense. I say that because up until last year I had been using a dichlor product to shock. My understanding is that the dichlor overstabilizes the water, which would leave more CYA to decompose over the winter. Then in the spring, when the CYA has turned to ammonia, the pool would require massive amounts of chlorine to clear the water. I would then use more dichlor shock to satisfy the pool's need for chlorine and eventually the water would clear. But the use of so much dichlor would raise the CYA level so high that it would set this whole cycle into motion again once the pool was closed in the fall. Is that a plausible explanation of what's going on?