Algae [black?] Stains


New member
Jul 1, 2010
Dalllas Tx
FC - Free Chlorine = 3
CC - Combined Chlorine [don’t know]
pH = 7.5
TA - Total Alkalinity = 95
CH - Calcium Hardness s [don’t know]
TH = 550
CYA - Cyanuric Acid (stabilizer) = 40

Pool size = 24,000
DE filter  Nautilus 60
I G pool

I have read TFP for 2 months and my pool has never looked better for crystal clarity but there is a stained wall problem. I have attached a picture of the stains.

I read the BBB method but so far it has just been B [bleach] because Ph is rock solid.

We went on vacation for a moth this spring and when we came back there was algae [black?] on the walls. I shocked it and applied True Green and killed the algae. The walls seem to be stained and it won’t brush off.

There is some pitting and I have even rubbed a Chlorine tablet on some spots and it improves the situation some but it seems to be as good as it gets. I tried a vitamin “C” tablet and it seemed to help slightly but no miracle cure. I don’t want to drain the pool and/or do an acid wash. I am afraid the acid wash will damage the plaster.

If my memory serves me right the sides and bottom used to sweep right up with the steel brush but it seems permanent this time. Brushing seems to do no good.



Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
It's guesswork from the picture but it appears to me you have calcium scaling on your pool and that dirt/algae has taken up residence within the scale. That's gonna' be tough.

Sooner or later you will have to deal with the scaling issue and remove the hidey holes where the dirt/algae is accumulating.

You could take your FC up to around 10ppm and HOLD IT THERE CONTINUALLY for a couple of weeks and see if that penetrates deep enough to get rid of the algae but, from the looks, it appears some dirt has been encapsulated within the scale so, long term, you will have to remove the scale to get totally stain-free.


TFP Expert
LifeTime Supporter
Feb 23, 2008
Denton, TX
That's what it looks like to me too (Ca scale) and this has given any algae some extra surface area to hide in.


New member
Jul 1, 2010
Dalllas Tx
I suspect I will have to re-plaster the pool to remove the pits. Right ?

I have gotten bids of $3,000 for plaster and $6,000 for some high end stuff guaranteed for 6 years or so. I have owned the pool for 17 years and I haven’t re-plastered.

I tried to locally raise the chlorine level overnight to bleach the stains with limited success. [See pictures.]

It seem like it would be much cheaper to shock and hold the free chlorine high for a week or so in January. Right ?

Before reading TFP I was shocking [to CL = 5+] once a week or so. Six scoops of trichlor 65% and the chlorine was costing a bundle.

Three cups of bleach a day seems to maintain Free chlorine = 3 and costs 24 cents.

Sam's club has janitor chlorine for $5.75 for 4 Gallons.



TFP Expert
LifeTime Supporter
Apr 4, 2009
With CYA at 40 and FC of 3 is your minimum. You should be targeting and FC of 5ppm.

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