You can use bleach as a sanitizer to keep the CYA levels down--that's exactly what most folks on this forum do!! You can use the "shock" left from last season even if you're using bleach, but read the ingredient label first--if it's sodium dichloro.........(long, unpronounceable word), then that also contains CYA, actually more pound-per-pound than trichlor does, if I'm not mistaken. It still can be used, but keep and eye on the CYA. If the ingredient is calcium hypochlorite, then it also can be used, is not stabilized, but will raise your calcium levels.