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

I have new neighbors and have been helping them open their pool to get a few weeks of swimming in before they have to drain it to replace the liner in the spring. The previous owners never opened the pool this spring knowing they were going to sell the house. Here's the story.

The pool is a 18' x 52" round vinyl AGP (about 7500 GAL). When the tarp came off two weeks ago, the water was more like sludge than water, and only about two feet deep. My neighbor filled the pool up the rest of the way, and I told him to start off with one 1.4 gallon bottle of 6% bleach. The green started to dissipate almost immediately, and was gone by the following morning.

He bought a DPD test kit from Walmart, and about 20 bottles of bleach on my recommendation. I also had him get some CYA at Walmart, and some Borax.

Initially, his pH was below 6.8, so I had him dump a box of 20 Mule Team, which brought him up to 7.6. His alk is at 170, and we brought his CYA up to 40. The water remained cloudy, however, almost milky white. He's been running the pump (severely oversized pump and undersized sand filter) 24 hours/day.

I had him vacuuming and back flushing daily, testing his chlorine and pH levels three times a day and adjusting as necessary. He kept it up, has been losing chlorine like crazy, and keep adding to shock level.

Now the water is clear down to about a foot above the floor, but milky white under that level. His chlorine usage has slowed considerably, until last night.

Today, the water tested back down below 6.8 pH and chlorine went to 0. WTH caused this? He has done nothing different, and has been diligent at testing and adjusting his chemistry every day, so why the sudden inexplicable drop in pH and chlorine from about 13 to nothing in such a short time?

I'm at a loss, and my neighbor is starting to get antsy. He's starting to think that I am driving him down the wrong road and that this BBB business is for the birds. I'd like to help him clear up this mess if at all possible, but I'm at a loss on this one. I did tell him to put in another box of 20 Mule Team and another jug of bleach, and we'll see what things look like in the morning.

Any further advice would be most welcome.

Numbers before the chlorine and borax add this evening:

TC = 0
pH < 6.8
TA = 170
CYA = 40ish

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.

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.