1. Regarding the shock you used, can you find it on the Walmart site, and give me the link? I can't tell anything unless I can get an exact chemical breakdown.

2. I doubt you'll need to drain because of CYA (stabilizer). In any case, at this point, we don't even know that it IS high. The HTH 6-way can test CYA accurately, but only 3x. So read the instructions carefully, and be sure you have a clear idea of what you're doing.

And, even if the CYA is high, there are ways to compensate quite nicely. The fact that this year Walmart is selling the 10% pool bleach makes that even easier.