Pool is clear but bottom looks spotty and dirty

[rkrgk]

I had Algae issues last season and followed the advice this forum gives and it has been going well. I dont know if I started noticing more this season or that there is something going on with my pool.
First of all, I have to admit that I dont know what material my pool is made of. Usually I use a plastic brush to brush algae off the walls and the bottom, but since I saw lot of algae buildup last two months and they were so stubborn to remove with a normal plastic brush, I bought a wire brush and used a bit on the walls. I was a bit scared to use it liberally since they seem a bit harsh on the surface. But since I really dont know what the surface is made of I am not that confident about using them. I am attaching a picture with this and if someone could identify what type of surface my pool has, it will be really helpful.
There are lot of black dots/patches at the pool side and the bottom and I felt that after I brushed with the wire brush, I see black particles coming off the wall, I thought they were part of the pool surface (assuming the pool is some type of stone type material), it could be algae as well.
So, is it really okay to use the wirebrush on my pool? will it scratch or destroy the surface, is the black spots really because the surface is disintegrating? Or is it black algae?

[BigDave]

Can you give us a complete set of numbers from the pool water especially CYA, FC, CC, pH and TA?

You'll really be lot happier if you invest in a K-2006 (or perhaps the k-2006C) because I think you might need to reach some pretty high shock levels to get this cleaned up but we won't know much until we have some reliable chemistry from the pool.

[rkrgk]

Can you give us a complete set of numbers from the pool water especially CYA, FC, CC, pH and TA?

You'll really be lot happier if you invest in a K-2006 (or perhaps the k-2006C) because I think you might need to reach some pretty high shock levels to get this cleaned up but we won't know much until we have some reliable chemistry from the pool.

Yes, I do have the Taylor kit. Here are the numbers, took just now.

FC - 3.2
CC - 0.4
CYA - Almost 30, but not quite.
pH - 7.6
TA - 125

I use the Clorox bleach from Costco and they recently upped the Sodium Hypo % to 8.25 and made the jugs smaller(121 fl oz). I seem to use lot of bleach but seems like I cant quite get rid of the Algae, and thought may be I am not scrubbing them off enough from the walls. Dont know if using the wire brush can damage the surface. The water is clear, but the surface appear to have lot of patches (seems they are algae).

[rkrgk]

Adding another closeup pic

[BigDave]

I can't tell you if it's OK to use a wire brush - sorry.

It looks to me like the black stuff is something growing that will need to be killed. The fact that you've had algae recently, you have a measurable CC, and you feel you've been using excessive amounts of bleach supports the idea that you're fighting some bio-growth in the pool.

I'd recommend that you shock the pool. Are you familiar with the shock procedure recommended here? If not, a recap:

Raise the FC to shock levels (for your CYA level as per Ben's Best Guess Chart - use the 30-50 row as you're close to 30).
Test and restore FC to shock level as often as you can and don't let it fall below shock level.
Run filter 24/7.
Brush as often as you can (at least daily). The brushing is to expose the black stuff to the chlorine, not necessarily to scrub it all off.

Continue this process until one day after:

The pool is clear.
The pool loses less than 1 ppm FC between sunset and sunrise.
There's nearly no CC (less than 1 drop of R-0871 after adding the R-0003).

If there are no swimmers, maybe try the +shock+ level. I understand that black algae (if that's what this is) can hold strong to the tiny pores in concrete.

One other thought; If you use a 10 ml sample (multiply by 0.5 ppm instead of 0.2 ppm)for the FC and CC tests, your testing reagents will last longer.

[rkrgk]

Thanks BigDave.

FOr 8.25% bleach 121 fl oz jugs, can you tell me how many jugs I should pour each evening?

I am still concerned whether to use the wirebrush or not, they seem very effective, but if it is at the cost of damaging the surface i would be worried. On the other hand, the plastic brush seem not doing the job for me, Ive been using that.

Thanks again!