Annoying black algue stain that cannot be brushed at the bottom of the pool?

[marc.tremblay]

Hi all! I've been maintaining my above ground pool for 4 years and I have never needed help maintaining a crystal clear water and superb liner, but now I feel like I do for the first time.

My specs: 21 ft above ground pool with vinyl liner. Typical residential sand filter and pump.

There is an approx. 4 ft2 black stain that appeared in September last year at the bottom of my above ground pool. I thought that after winter it would go away with everything I do to open the pool in the spring (cleaning, brushing, shocking, adding half of the pool's volume water) ... but no, it's still there.

Here is everything I know about this issue so far:

1) Brushing this (even applying force when brushing) does not change a single thing. It's not coming off AT ALL.

2) I then read online that it could be some oxydized metal particles depositing there, so I purchased and used 250ml of this Stain&Scale bottle, but it didn't change a single thing, so it looks like it's not a metal stain:


3) I also read online that it could be black algue and that directly depositing granulated chlorine on top of the stain using a pipe and waiting a couple hours could help. It COMPLETELY removed the stain! 2 weeks later, the stain re-appeared, same location, same size, same color, same thing all over again. Tried this once more and it exactly yielded the same results again, it disappeared for 1-2 weeks and then re-appeared same size, same location.

Am I doomed? Is there a way to permanently get rid of what seems like black algue?

Manganese? (maybe Stain&Scale does not solubilize manganese?)

Thanks!

[Denniswiseman]

Try some Vitamin C tablets left on it and see if it moves where the tablets are
If it does you need to do an Ascorbic Acid Treatment

[Teapot]

Hi Marc,
Please post a picture if you can.
From your description it sounds like black algae, only it isn't an algae. Black algae is actually cyanobacteria Yet another thing the pool industry has got wrong, they won't correct it because it leads to more sales! Cyanobacteria is clever enough to take up residence in plastics and plaster being almost chlorine resistant. You can try treating the bacteria with strong chlorine dioxide if you can find some, maybe treating with germicidal UV may help (not tried UV) but the cyanobacteria can lay almost dormant and re-appear if conditions become favorable again as you have found.

[marc.tremblay]

I'll post a picture when it reappears in 1-2 weeks (IRC and if conditions stay the same as last fall?). Worst case scenario, until I change the liner, I'll have to periodically apply granulated chlorine on top of the stain when it shows up once every 2-3 weeks I guess. I guestimate it's a 30$ chlorine extra per summer and 2 minutes per month extra But I'm curious to know exactly what this reoccuring and absolutely stubborn stain is.

[Teapot]

I'll post a picture when it reappears in 1-2 weeks But I'm curious to know exactly what this reoccuring and absolutely stubborn stain is.

As I stated earlier, it is most likely cyanobacteria, (black algae wrongly described by the idiot pool industry)

[marc.tremblay]

Teapot, may I ask more information about this hypothesis? I have a question: Wouldn't UV rays from the sun hit the bottom of the pool on the liner and kill that bacteria if it were the case? (my pool is exposed to sun rays ALL DAY LONG, no shade at all)

I also read online that cyanobacteria is cyan/green. The stain that always comes back in my pool is pitch black, not green. EDIT: Also read on another web site that cyanobacteria is black. WTH... I read that it's more common in plaster pools, but mine has a vinyl liner. Also, my pool has a vigorous water flow similar to a toilet bowl, with the typhoon in the middle, above the floor drain. I read that cyanobacteria is usually found in calm water. Not the case in my pool!

[Teapot]

Sadly none of what you mentioned has any real baring on the case. No the sun's UV doesn't penetrate that far and the germicidal frequency of 254nm is sadly absent otherwise no algae would grow in any pool.
If you post a picture then maybe we'll get a better idea, it's not easy giving prognosis from the end of a keyboard. Your symptoms sound like black algae and on another thread on here you can see it growing nicely on the underside of some pool ladder treads.

[marc.tremblay]

EDIT July 28th: It's been 8 weeks since this post and I only had to deposit chlorinating granules on the stain twice. It's now been 4 weeks since the last chlorinating granules deposit and my pool is still free of the stain. I think as long as I keep the pool properly chlorinated and never miss out on putting chlorine pucks when older pucks run out, the stain won't reappear. It only appears if I forget to put new chlorine pucks for a day or two. Well, some day I'll change the liner anyway, so I'll put in completely new water on a new liner and that black algue stain will disappear forever, I guess. This is solved, I guess. Thanks.