Or, if you are low on CH, or not so high that it matters, you could consider cal-hypo. In the big buckets it is cheaper than bleach and you can shock the heck out of the pool really fast. I like to keep it on hand because it stores so well. When you need to shock, sometimes the amount of bleach you need can be a problem to get a hold of fast and at a good price. But if you have a pool store that has 12% bleach that may be even better overall.