Black "oil looking" stains

[tgenre]

HELP! I have a few black spots on my plaster that look like drops of black oil. I'm 99.9% sure I had nothing drip into my pool. I've had a couple of these for months that I ignored but now I have several that I think developed over the weekend. Is this common? Do you have any idea what it is? I've had the stains that come up with Vitamin C, but this is not that.

FC 4
Ph 7.8
TA 70
CH 525
CYA 65
Salt 3300 ppm
Pump runs 10 hours/day with SWG at 50%

 

[Richard320]

Can you scrape it off with a fingernail? If so, do that and rub it on a piece of white paper. If it's actually dark green, it's black algae.

 

[tgenre]

Will not scrape. And thanks for responding so quickly.

 

[tgenre]

Any other suggestions?

 

[JamesW]

Can you post pictures?

 

[tgenre]

At work now...will take pics tomorrow and post

 

[Butterfly]

What type brush do you use and how often do you brush the pool? Does brushing lessen/lighten/make any difference with the spots?

 

[tgenre]

I do not brush often enough. I've never had anyone tell me how often I should brush (other than the initial twice daily brushing with fresh plaster). It is a Nylon (I think) brush. I probably brush the pool a few times a month, to be honest. The spots have actually lightened (although they wouldn't scrape with a fingernail or respond to vitamin C). They are still visible, and I have some that have been there for a while in the deeper end - those are still dark. I'm not sure about the metal content in my water, as I've not had a pool store test my water since being a TFP member!

 

[Butterfly]

Brush recommendations:

Nylon is for vinyl, fiberglass, tile, and plaster that is less than one year. (Plaster includes any exposed aggregate and marcite finish, btw!) Combo brushes are for older plaster. All stainless are for algae (very stubborn green algae and also the ONLY brush for black algae.)

You CAN unscrew the Wall Whale from the nylon brush it comes with and screw it on a combo brush if your plaster is over a year old. Newer plaster is really too soft for anyting other than a nylon brush.

Try rubbing a trichlor puck on the stain, or let it sit on the stain for a minute or two and see if it lightens.

It would be good to brush the pool once a week. Most of us probably do not brush often enough.

 

[tgenre]

Thanks for the education. My plaster is 6-7 mo old, so I'm good with Nylon brush only at this time (I just need to use it . I'll try to get some trichlor tabs and see what happens.