sludge/goo behind inground pool light

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

Yep, shock shock shock. Run the pump 24x7 while shocking. do your best to brush the areas like the niche if you can until the algae dead and gone. The high CYA combined with not much flow in behind the light into the niche area are probably the cause of the problems. As long as water is able to flow into the drains goo is coming out of, I am pretty sure shocking with a manageable CYA level will take care of those. Make sure to manage your pH, since it was already over 8 and your water has 450 CH. Scaling is likely in that combination.

I'm not sure if it's safe (or worthwhile) to try and get something physically into the algae filled drains to help things along. Hopefully someone else will comment.

Your CC will probably climb back up some as the chlorine does its job.

Thank you kelemvor. I finished with the water refill, adding what bleach I had left to be proactive when I was filling it.

New readings (in daytime sun, mind you) are

fc 13.5
cc 0
ph 8+
acid demand 2
ta 100
ch 220
cya 60 (yes, 60!!!)

So, do I go shock to the mustard algae level of 34 or shock to normal level of 18? What do you suggest? Thank you for your help. I am grateful.

Update: I went with the Mustard Algae Level. The pool was cold and I figured I had about 2 weeks before it would be warm enough to swim in. Since then, I have remained algae free.

On another note, I have been doing what I learned on this site, testing daily with the Taylor Kit, giving the pool what it needs every day, taking care of weekly maintenance, using BBB method. The pool has been crystal clear for a month. The next door neighbors can't believe how clear it looks. Their daughter saw it and said "Its clear and blue! I don't believe it. I have never seen it like that!" It seems the previous owner never got past a green tint, even if he did clear it once in a while. So, I am saying thank you to Pool Forum. I never would have gotten to this point without the help I have received here, both in my understanding and in results. My kids have used the pool every day for the last 2+ weeks. They love it...and the pool has been algae free and balanced for a month, which makes me a happy camper.

FC between 5-9 every day
CC between 0-1 every day
PH 7.7 every day
TA 90
CH 240
CYA 60

Thank you!!!!!

It is good to hear another satisfied member telling of how much he has learned about maintaining a pool. Thanks for your post and hope you have a great swim season.

By the way, as far as I am concerned, there are two seasons -- "swim" season and "can't wait til it's swim season again" season. ;);)