Black Algae or something else?

[shayanhe]

I have a 30,000 gallon in-ground pool with vinyl liner. I have had a stain in the pool for a while now. I have tried the usual stuff (AKA Vitamin C tablets, Natural Chemistry Stain Remover, brushing) and nothing has worked on the stain. The stain's color change throughout the day. It appears a lot lighter around noon when the sun is out. I have noticed that the stain is also slowly expanding. I would really appreciate your help in identifying the stain.

 

[JVTrain]

Have you tried holding a trichlor puck against the stain for 30 seconds or so? Too long and you could fade your liner but if it fades with a trichlor puck on it, it's an organic based stain, aka algae. Possibility of something behind the liner? Is this on the bottom of the pool or a wall?

 

[shayanhe]

I have tried holding a trichlor puck, but that did not help. It could be something behind the liner. This is on the bottom of the pool. Is there any way to eliminate this or stop it from spreading?

 

[shayanhe]

Can you guys suggest a way to remove this stain from my pool?

 

[JamesW]

Probably coming from behind the liner. No good fix without removing whatever is under the liner.

 

[shayanhe]

Probably coming from behind the liner. No good fix without removing whatever is under the liner.

Is that algae behind the liner? If so, do you think I can kill it with placing a dark tarp over the pool to remove light?

 

[JamesW]

Usually bacteria or fungus. Only way to get rid of it is to physically remove it or chemically treat it directly.

 

[Swampwoman]

Shayanhe, if it is a growth behind your liner, its possible you have a small, possibly pinhole leak thats allowing water to sit behind it, trapped, and the FC wears off.

I've not known a poster directly whose tried this, but have see reports that in some cases, treating soil surrounding the pool area with ferric sulphate and watering it in (avail at agricultural outlets) may kill the fungus over time.

Slamming ( SLAM Process) might temporarily reduce it, but not resolve it long-term.

If a liner change is in your near future, they can bleach-treat the floor, and install foam to prevent future communication if you know it has this tendency.

In the mean time, if the stain does not respond to anything you try on it, you might consider one of thse pool floor decals to conceal it if its in the kind of spot where it makes sense.