Why Can't We Get Rid Of This Dark Green Stain

by Angie

Our pool has a dark green stain on the sides and it just won't come off. When it appears that we made a difference in the condition, the next day it is back again.

On the edges of the floor there are spots of what looks like dark sand, it is stuck to the ground and won't come off if we scrub or vacuum.

We tried some different pool chemicals that said it would get rid of it but it didn't show much difference. I was hoping you could identify the problem and quick solution.

It is rather embarrassing when you want to throw a party.

Please and thank you!


Thanks for the question Angie

First, I'd like to have your complete chemicals readings:


CYA (cyanuric acid/stabilizer)



Calcium Hardness

Metals (iron and copper)

Total Dissolved Solids

It makes troubleshooting much easier and the process of clearing up your pool will go much faster.

You can get this done at your local pool store.

Without these numbers I'm just guessing at what the problem is.

Next is to have a good test kit. I'd advise you to get a Taylor FAS-DPD K-2006 pool test kit.

Pool Water Testing

Water Testing Kit

You need to see if the stain or organic or mineral.

Using a Trichlor tablet, hold it on the stain for about 10 minutes. Use a stick or a pool pole, not your finger.

If the stain lightens up or dissapears, it's organic.

You can also use about 1/2 lb. of calcium hypochlorite (granular chlorine) in an old sock to put on a spot.

If the stain doesn't lighten up, gets darker or the color changes to black, it's probably a metal stain.

To confirm it's a metal stain, get a Vitamin C tablet (crushed up acsorbic acid), put that in a sock, and place it on the stain.

If the stain is removed, you can treat the pool with ascorbic acid.

A detailed ascorbic acid treatment can be found in this post:

Brown Algae Won't Come Off Of A Vinyl Liner

Get back to me with your readings and we can go from there.

To post a reply, or if you have a similar question, you can see your post (Why Can't We Get Rid Of This Dark Green Stain) on the Q&A page in the "Algae/Green Pool Water" category.

Swimming Pool Questions and Answers

Check back to this post for answers.

Hope this helps and have a great Summer.


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Pool Algae Help

by Ray
(San Jose, California)

I have an inground plaster 20,000 gallon pool, cartridge filter. I live in northern California, so no extreme winters or summers. my neighbor has huge trees that just dump leaves into my pool, but I cant do anything about that.

I have been struggling with my pool for 6 years, I've had 2 different pool guys but had to let them go due to money problems, then I try to maintain on my own which works for a while.

Every time I think I have the maintenance down something happens and my pool is green again. I've even considered filling the pool with dirt to save frustration. I have recently tried using the "pool perfect plus phosfree" from natural chemistry, in addition to weekly shock treatment, but for some reason I still get algae.

I run my pool filter for 8 hours a day. what else can I do? should I be using algaecide? my pH levels are good, I use pH down when needed. Should I run my filter more than 8 hours a day? Thanks for any help

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