If it were my pool with a cya reading of 100 and who knows how high it actually is (like Waterbear said, can't differentiate between 100 and >100), I would do a partial drain and refill and try and get the cya down to a more manageable level. Depending on high how your cya actually is, it could take a couple of partial drains and refills. I think I would drain about 1/2 or maybe 2/3 of the water and refill and then retest and see what the levels are then. Just my opinion. At any rate, regardless of whether or not you decide to do a partial drain and refill or not, you should NOT use any more trichlor. No dichlor powder either.