Neighbor has cloudy pool after 10 days of BBB, sudden dip in pH, TC

[mbar]

What is the shock level he has gotten to? Has he been brushing daily? I think what happened is that the algae was not completely killed, and it took hold again - possibly if it rained, or more water was added. If the chlorine level wasn't at least at 15 at all times, the algae could take hold again. When he started, he had no cya, as the cya came up, the chlorine wasn't as powerful as it was first. This also gave the algae room to breed. As for the ph bounce, I would have him turn his return jets up so that the water aerates, this will bring down his alk while raising his ph.

My suggestion would be to take his chlorine up to 20ppms, even to 25 to really kill the algae. Keep the filter running 24/7, brush, brush, especially the bottom, and turn the jets up so the water aerates. Put a skimmer sock on the skimmer to help catch the dead algae. Brushing the bottom where the water is milky should help to stir up the dead algae so it can be filtered out. Most important is to keep the bleach at high levels - with all the algae, he may need to go higher than the shock level for cya of 40 is - so I would take it up to 25ppm.

[Watermom]

Many of us on the forum with sand filters routinely (after backwashing) add a little DE to our filters. It seems to help the filter catch more stuff. You only add enough to make the pressure on your filter rise by 1 psi. Premix it with a little water before pouring it in the skimmer. It will usually take 20-30 minutes for the pressure to rise, so wait a bit after the initial dose before deciding if you need to add a little more. It might help clear your pool faster.

Also, normally I wouldn't recommend shocking the chlorine up that high with a cya of only 40, but since the original poster said that the liner was going to be replaced in the spring anyways, in this case it is probably fine to do so.