Pool is clear but bottom looks spotty and dirty

[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!

[BigDave]

121oz. of 8.25% bleach should add about 5ppm FC to your 15K pool.

How much to dose depends on the measured FC level and how much you predict the pool will lose before the next FC measurement / dose.

Let's assume a 3ppm FC loss in a day and the last measurement is 3ppm FC. Three 121oz. jugs of 8.25% will add about 15ppm FC raising the FC to 18ppm. if the pool loses 3 ppm tomorrow, the remaining FC should be 15ppm - the minimum shock level. Then add a bit more than a half a jug to get back to 18ppm so the FC will be at least 15ppm the next time. NOTE - this is only an example - you'll have to put in your pool's loss rate

It's important to remember to measure - dose - measure and adjust as necessary. The more often you do this during the shock process, the better it will go.

As for the wire brush - you ultimately have to make that call. perhaps someone here will recognize your pool's surface and chime in but I have a vinyl and wouldn't go near it with a wire brush.

[rkrgk]

121oz. of 8.25% bleach should add about 5ppm FC to your 15K pool.

How much to dose depends on the measured FC level and how much you predict the pool will lose before the next FC measurement / dose.

Let's assume a 3ppm FC loss in a day and the last measurement is 3ppm FC. Three 121oz. jugs of 8.25% will add about 15ppm FC raising the FC to 18ppm. if the pool loses 3 ppm tomorrow, the remaining FC should be 15ppm - the minimum shock level. Then add a bit more than a half a jug to get back to 18ppm so the FC will be at least 15ppm the next time. NOTE - this is only an example - you'll have to put in your pool's loss rate

It's important to remember to measure - dose - measure and adjust as necessary. The more often you do this during the shock process, the better it will go.

Thanks! Yesterday, I did exactly what you said. I added 3 jugs and I analyzed the water after an hour. The data showed exactly what you said. Since I got it above 15ppm I thought that was good.

FC - 18.5 ppm
CC - 1.0

I analyzed the water again this morning and as you said, it got reduced to

FC - 13.5
CC - 0.5

Almost 5ppm loss. So I added a jug more awhile ago. And brushed the walls.

As for the wire brush - you ultimately have to make that call. perhaps someone here will recognize your pool's surface and chime in but I have a vinyl and wouldn't go near it with a wire brush.

I used the wirebrush anyway and seems like there is some improvement today (even though there is lot more to do). Probably I have a gunite surface. yeah, hope someone could confirm here

Another question, does adding bleach increase the pH, I think the pH increased to about 7.9. Or is it that the test is giving me false value at high FC level?

Also, should I add a bit more CYA ? And is CYA test values accurate at high FC level?

[BigDave]

That is good but try not to let the FC go below 15ppm.

Shock levels will give falsely high pH readings - at 7.9, I wouldn't try to move it until the shock is over and FC returns to normal. I also wouldn't move CYA until the shock is over. Some people in warm, sunny areas like to run the CYA up to 90 to reduce total FC consumption and the frequency of adding chlorine - just remember to raise the minimum FC to correspond.

The black stuff may be easier to brush away after it's dead, just try to brush it enough to keep it exposed to the shock level water.

[rkrgk]

No new information, just to update that pool is looking a bit better, may be halfway through. Already backwashed the filter couple of times (pressure increased overnight to +6) and the skimmer (fine sieve) is catching lot of live algae(green). But loosing 5+ppm of Cl overnight. I guess it might be a better to add a bit more conditioner since I have below 30ppm CYA and the sun is brutal in Phoenix. And need to order more Taylor DPD-Titrant reagent as well before it gets finished. Thats all.
I am really interested to know how people remove algae without a wire brush. Big difference for me.

[BigDave]

If you do decide to add more CYA, remember to raise the FC targets as well (for shock and regular maintenance).

[robbym70]

I am really interested to know how people remove algae without a wire brush.....

My set up is totally different than yours but when I need to scrub the walls of my vinyl pool I use a stiff nylon brush. I thought maybe switching to a stiff nylon brush might be a good alternative for you if you think the wire brush is damaging the surface of your IG pool. Couldn't hurt trying it out.