Stabilizer is NOT the same thing as a clarifier. Clarifiers cause smaller particles to clump together so they can be filtered out more easily, however, we seldom suggest using them. Stabilizer, or CYA, is kind of like sunscreen for your chlorine. Without enough in the water, your chlorine is quickly lost to the sun.

Regarding the two shock columns, you typically only need to shock as high as the first column unless you have a really resistant algae like maybe mustard algae. However, if you keep your chlorine at the proper levels, you never have to shock. Some pool stores will tell you that you need to shock weekly. That is not true. The only time you ever need to shock is if your chlorine levels drop too low or if you have a lot of organic debris in the pool. I seldom shock. I test my water almost daily and always maintain proper chemistry. When you do need to shock, you can just use bleach. We like to say that the word "shock" is a verb and not a pool product. You would just add a larger than normal dose of bleach to get to the shock level that you need based on the Best Guess chart. Make sense?

FC + CC = TC. Always. Regardless of what your CYA levels are, the equation is always constant. Ideally, you want your CC to be zero which means that your free chlorine and your total chlorine would be the same.

We may have already told you this but not sure. But, in your 25K gallon pool, each 121 oz. jug of 8.25% bleach will add about 3ppm of chlorine to your pool. Use that as a reference to help you figure out doses of bleach that you need to add to your pool.

Hope this is all helping you better understand taking care of your pool chemistry.