Most effective and cheapest. The most common practice is to use tablets before/after bathing. Depending on how much you bathe, you shock-chlorinate instead of the tablets after bathing, every two or three weeks.
The shock chlorine removes chloramines (spent chlorine) and other contaminants. Very good to occasionally drain some water and refill with new "spa water".
If there are bacteria and/or organic matter in the water, the chlorine and chlorine atoms will "eat" these contaminants. Thus, the chlorine is consumed. It is not uncommon to find that, through this process, the added chlorine disappears immediately.
Add the chlorine. Wait 30 minutes after this and then measure the chlorine value with the test equipment. If you again "measure 0" for chlorine, add more chlorine.
Once all contaminants have been removed, only then will the chlorine remain in the water and the chlorine level will be in the recommended range.
