Calcium scaling or black algae


New member
Jul 24, 2011
New to the forum, but I have been reading most of the basics, and trying to come up to speed.

I am seeing a strange dark stain at various parts of my pebble-tec pool. Thought it was black algae, but after looking at posts ont his forum, I thought that this was probably not the same thing. However, I was already taking steps to combat black algae, so I am in the midst of shocking the pool and scrubbing the dark areas. I saw pictures of calcium scaling and thought that might be what is actually going on.

Here is a picture:
[attachment=0:3j2exj10]Pool staining 2.gif[/attachment:3j2exj10]

Did measurements on the pool using the TF100.

FC: 6.5
CC: 0.5
pH: 7.2
TA: 110
CYA: 25
CH: 1100 (Wow!)

I didn't do measure the CH until today (after shocking for 3 days), after reading about the scaling.

So any ideas? Is this scaling? Everything else looks good on the pool except for this dark discoloration (or is it bleaching of the other areas).

Thanks for any help.



Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
Welcome to the forum. :lol:

The picture is too vague to be definitive but it doesn't look much like algae. That said, your test results (assuming they have been fairly stable) would not indicate algae, either, but would suggest scaling.

What is the CH of your fill water? Has your pH drifted up higher than that from time to time?

Slightly off the subject, but those results say you are not shocking your pool which would require an FC up around 12ppm or so with a CYA of 25ppm.


New member
Jul 24, 2011
The pH always seems to be drifting higher. This measurement was a few days after pouring a gallon of acid in and bringing the pH down to 6.8.

I just measured the calcium of the fill. I took the water straight out of a spigot near the pool, because I didn't know how access it directly. I assume these are the same as long as they don't go into the water softener.

I measured the CH of the water to be 420. Again, wow! I am in Southern Cal, and I know the water is hard, so I must be seeing the result of it.

So, will I need to drain the pool to bring the CH down? What do I do about the high levels in the fill?


Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
Tucson, AZ
Reverse osmosis is the only option to remove the CH. Best case with replacing water would get you near 500ppm. There are others on the board that are successful at maintaining their pools with 4 digit CH. You have to be VERY careful to keep your pH in check.

Posted with Tapatalk ... sorry if I sound short ... hate typing on phone :)

Melt In The Sun

TFP Expert
LifeTime Supporter
Oct 29, 2009
Tucson, AZ
It might not be algae or scale, just some variability in the surface of the pool. Can you get a close look with goggles? Black algae is little fuzzy bumps that obviously protrude out from the surface. You can scrape them with your fingernail and they're green.