Two Questions (Sanitize and Empty Tank)

I am sure there are chlorine tablets you could buy, but why? You would never know if they are fully dissolved so you wouldn't know when it is safe fill the lines or to drain the system back down. Bleach is cheap and easy. Add bleach, fill tank, fill lines, let set for recommended contact time, drain down. That seems pretty straight forward. Why do you want to use tablets?

I do mine once a year at the beginning of the season.