CC is produced as FC is consumed killing something in your water. With a CC of 2 it is almost guaranteed that you have some level of algae. You need to bring your water to "shock" level and keep it there consistently until you are measuring 0.5 or 0.0 CC. You'll know you've killed off the algae when you can go from sundown one day to before sunup the next without losing more than 1ppm FC.

According to "the chart" you need 15-30 FC for shocking a pool with a CYA of 40. In a 23,000 gallon pool that's at least 6 gallons or 8.1 (96 oz) jugs of 6% "ultra" bleach to reach 15ppm FC.

It's important that you test your FC level and add chlorine often to maintain that 15ppm. 2-3 times a day if you can do it. Usually I go a good bit beyond the minimum for shock (I'd go to 20ish) but with a vinyl liner you can bleach the liner and end up causing it to lighten.

As for the dippers of powder. If the sample turns pink with one dipper you can stop adding DPD at one scoop. If not, add the second. I would recommend always using the 10ml sample size. It uses less reagents and .2 precision is not useful at all compared to .5 precision.


Do not add any more CYA or adjust anything else until you've completed the shock process. Some of the tests can be affected by high chlorine so you can't trust the other test results until you're done.