Was at a customer's pool. He thought he was not getting chlorine production from the T-15 cell that he had for 2 1/2 months. There was a thick layer of white flakes that didn't look like calcium and his AquaRite readings were within normal ranges. He was "sure" that his pool water had no issues. Returned to the shop and let the cell soak for 30 minutes in a muriatic acid/water bath. Some calcium came off right away as you could see the fizz however a vast majority of the material just sank to the bottom the bucket. Thoughts? Plaster problems? Tested the cell on my tester and it worked fine. Produced plenty of chlorine. Must have been so coated with this material that the chemical reaction from the plates couldn't happen.