Algae, Fungi, or have Alien hell-spawn invaded my pool???

Hi all, need some help.....

I've been dealing quite well with this pool (came with the house) since '03. Current readings are all within proper limits and all was well until about 3 weeks ago. Stains have developed on the bottom of my pool and they appear to have a cause that is coming from underneath the liner. They are random in pattern, are grayish tending towards black, started on the slope to the deep end and are slowly spreading. There is nothing to scrape or rub off, i.e. not scale or some tough algae; it seems to be dicoloring the vinyl although it has not degraded it. I called an old friend who has been in the pool biz for about 20 years and he told me that he had heard of it happening and knew of someone who had solved the problem chemically but hasn't been able to find out what that specific solution was. Other cases he knew of had more serious outcomes including opinions that the soil beneath the pool was "bad" and was causing a fungus that could come through the vermiculite beneath the vinyl liner. This resulted in a couple of solutions that were pretty wild solutions including pulling the liner and soaking the vermiculite with bleach or worse, replacing 2-3 feet of soil and then redoing the bottom of the pool neither of which guarantees success and will cost major bucks. I am hoping someone has run across this and has a more economical (and less traumatic) solution......Thanks for any ideas, Rob

The Mermaid Queen

TFP Expert
LifeTime Supporter
Mar 28, 2007
Northern KY
First of all, welcome to TFP!

To help you properly, we need more information, especially your latest numbers, and what kind of test you are using (strips, drops, pool store). Also what method of sanitation are you using? (bleach, tri-cholr pucks, baquacil, Nature2, etc)
Thanks for the prelim, will put up current readings tomorrow but I use strips in between pool shop readings to check chems. For sanitation, tri-clor pucks in a chlorinator with periodic shocking based on use levels....Rob


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
Mold, fungus, or algae growing under the liner can be quite unsightly but are unlikely to cause any real problems. Some kinds of plants can grow right through the liner but those ones won't. All of the solutions require access under the liner, which is likely to be a real pain.


TFP Expert
LifeTime Supporter
In The Industry
May 20, 2007
SWSuburban Chicago, IL
Yep, mold under the liner, or my good friend-black algea. He's here to stay and I've learned to accept him. The only thing that helps is shock level of chlorine, the stain disappears, only to come back when the level drops. Oh well, not going to pay to replace the liner at this time. I've learned to accept it and enjoy my pool. Hopefully my friends won't judge me... :)
Ok, current numbers that I have look like this:

FC 30ppm
TC 30ppm
ALK 120
PH 7.3
Hardness 225

.....and yes, that is according to strips but I promise to get a grown-up test kit for better accuracy and more complete statistics. However, I had nice clear trouble free water for almost 4 years before developing this problem and only used strips backed up by pool store analysis. Could some long term deficiency be causing this problem????


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
Could some long term deficiency be causing this problem????
How have you been chlorinating your pool? Pucks? If so, your CYA could be thru the roof rendering your chlorine's a gradual process.

Black algae, while possible, is not a frequent visitor to vinyl pools and could be at least partially removed with a deck brush.
certainly sounds like liner mold to me. Caused by damp ground under the liner and the cure IS to remove the liner and sterilzie the soil underneath, uaually with chorine and lay new sand or vermiculite (and of course a new liner!)
Or, you could just live with it until you need to replace the liner and then take care of the problem.