Black residue on pool bottom and side walls

[Studiovette]

Hi my pool is 25000 gal with vinyl liner. It's abut 12 years old but this is my first year with it and 1st year with a inground pool so this is all new to me. My water is crystal clear but since I opened it I was getting residual light brown/tan residue scattered in deep and shallow end near the returns. I thought it was DE from my filter so I had pool co come out and clean and inspect my Hayward filter. After inspection they verified all the internals were good and no tears so that eliminated that possibility. So for a while after that the residue stopped. As of last week its back but now its actually black. As soon as I put the brush to it it dissipates. Gets a little cloudy and gone. Besides these 2-3 patches the liner in some areas is coated in black also and again as soon as I brush it, it goes away. As for my levels. So obviously with all this happening I decided to check my levels and noticed my alkalinity was at 30! And PH was like 6.8. Everything else was ok. I also checked for phosphates and they were 0. Just this morning I added about 25 lbs of alk plus to get that to the right level and see what happens. Does anybody have any other suggestions? I am very new to this and would appreciate your help. If I missed something which I probably did let me know.
Thanks

 

[Richard320]

Welcome!

If it looks like the dust bunnies under the bed but in water, it's most likely algae. My guess is when the pool guy checked the filter, he also dumped in a bunch of powdered "shock" which killed off most of it, but it also lowered your pH and TA because the stuff is very acidic.

You are correct in raising TA; if you used Baking Soda to do it, the pH will still be too low. Once pH is good, you need to SLAM it. That is, raise the chlorine level very high and keep it up there for several days until it passes some tests.

Please post a full set of test results and tell us where you got them. Then we can work with objective facts.

Have you visited Pool School?

 

[duraleigh]

+1 what Richard 320 said.

 

[Studiovette]

here is a pic of shallow end. I was thinking of using nylon sock on return to see if it catches anything. if its black algae what do you guys recommend?

 

[duraleigh]

That's not black algae, but it's almost surely dead algae.

Can you post a complete set of test results?