I opened my pool this week. After battling algae in the fall, I opened up to a clear pool.

Pool guys shocked pool after they opened it and I took readings 2 days later.

19.5 fc
.5 cc
8+ ph
5 acid demand
140 ta
450 ch
135 cya (guess, because scale ends at 100)

With the cya and the ch so high, I decided to bite the bullet and drain 40+% of the water and refill. As the water line came below my pool light, a black or dark green goo started seeping out from behind the light. I remove the light and a huge amount of this sludge fell into the water. I am guessing I could have filled a a plastic shopping bag with the stuff. I brushed it into the drain, as I was still draining the pool. Then more goo started coming out of another drain (?) on the side of my pool and into the water. It is quite nasty. I now believe this stuff is why the previous owner always had algae problems in this pool, among other things (pool needs to be refinished too).

So, do I do another all out shock when the pool is filled? Mustard algae level shock, or regular shock? I left the light out of the pool, so the water can get in the opening when the pool fills up.

Suggestions?

16X40 rectangular gunite pool; 24,811 gallons; Hayward sand filter S220T; Hayward super pump 1.0 HP