Black Mold On Inground Vinyl Liner

by Kelly

Hi Robert, I am coming back to the pool Guru for help. Last year you helped me through a phosphate catastrophe, this year I have what they tell me is black mold/algae under the pool liner.

I took my water in and below are the readings. I showed them a photo and that was their guess. With that said, if it is black mold, changing out the liner is not an option for us this year. My husband has not worked in 3 months, the summer weather is upon us and I am sure if I had the money to get a pool employee out to my home now would take months.

I need your help, if you have any to offer. They had me put in 2 pounds of Bioguard maintain burnout 35 fast dissolving shock.

I must say the stain diminished but is back again this morning :(...please help!

10,000 gallon, vinyl, built in kidney shaped pool
Temp: 80 degrees
Saturation index: 0.7
TDS: not tested
CYA: 29
Total Chlorine: 2
Free Chlorine: 2
PH: 8.1
Total alkalinity: 109
adj total alk: 100
total hardness: 304
iron: 0.1
copper 0
optimizer +: 0

Other then the staining on the liner, the water looks fantastic! lol

Thanks for the question Kelly and glad to hear from you again

First, black alge/mold is pretty uncommon for vinyl liners. It happens more with plaster. This is because it digs into the surface and creates layers for itself, like an onion.

Going through the readings, the CYA is just a bit low. Use Dichlor chlorine. This is a stabilized chlorine that will add CYA into the pool. Be careful when using Dichlor as it can get out of hand quickly. For every 10ppm of chlorine added with Dichlor, you'll raise the CYA by 9ppm. The CYA needs to be between 30 - 50ppm. The chlorine level is perfect. 1.5 - 3.5ppm is the range.

To shock a 10,000 gallon pool requires 1.3 lbs. of Dichlor. This will add 10ppm to the CYA. Try first using 1/2 lb. wait for 1 full turnover of the water, about 8 - 10 hours, then retest the CYA. Be sure to filter 24/7 during this process. Stop using Dichlor when the CYA gets to 30 - 35ppm and go back to liquid chlorine. Get a chlorine tab floater (about $12) and use Trichlor tabs. Never put tabs in the skimmer.

I hate to say here we go again with the pool store, but... Bioguard Maintain 35 is Lithium Hypochlorite. This is an unstabilized chlorine and, in effect, is essentially identical to bleach or regular liquid pool chlorine. It is a fast-dissolving powder. The downside is the high cost.

You need to get the pH down a little, to 7.6 - 7.8, even 7.4 would be good. Anything over 7.8 - 8.0 and chlorine loses its effectiveness very rapidly.

The alkalinity is fine. The hardness is much too high for a vinyl pool. You can keep the hardness around 100ppm and still be good. Unfortunetely the only way to reduce hardness is a partial or full drain and refill. If you haven't had any problems with the hardness being that high then don't do anything. If your fill water is hard there's nothing you can do, but if you're adding calcium to the pool, don't do it any more. Remember this for next year so there's no added expense by buying calcium that you may not need.

The SI is fine. Optimal reading is -0.5 to +0.5, but 0.7 is good.

Now about the black stuff. Are the stains coming from under the liner? Where are they located?

You can do some simple tests to determine what kind of stain it is. You can crush some Vitamin C tablets ( called "ascorbic acid" if the pool store has any) in a sock and put them on the stain for 1/2 to 1 hour. If the stain lightens up or is gone, it's a metal stain and you'll need to treat it with an ascorbic acid treatment.

If it doesn't lighten up? Get a Trichlor puck and put it on the stain. Use a stick or pole to keep it there for about 10 minutes. If that lightens it up or removes the stain, it's organic. This can be treated with a good pool shock and an adequate chlorine level. Black algae in a vinyl pool is possible, but not very likely. You can also use about 1/2 lb. of calcium hypochlorite (granular chlorine) in an old sock to put on a spot.

Don't leave the chlorine on the spot for too long as it may bleach the liner. 5 - 10 minutes max.

For a semi-bad part. The stain could possibly be a fungus and growing under the vinyl liner. To treat a fungus would mean getting a fungicide and putting that under the liner. This means draining the pool to apply the fungicide and installing a new liner. Hopefully it won't comes to that.

Here are some posts to look over:

Vinyl Liner Has Dark Stains On Bottom

Here's the ascorbic acid treatment:

Brown Algae Won't Come Off Of A Vinyl Liner

And the process for shocking the pool. It's the same method:

Green Pools

Try the tests above and let me know how it turns out for you.


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Black Algae on Fiberglass Pool

by Dave
(Burlington, NJ)

We have a fiberglass pool which in a matter of a couple days had black stains. I did your test with a 3" tablet & it is black algae. I thought because of the smooth surface it was harder for the algae to cling & form. I used 1 dose so far of black algae remover from Leslie pools & also have been brushing. Not sure yet if it helped. When I brush, I really can't see it doing anything. Sand filter & had water tested with readings in range. In fact chlorine was a little high, 7-8. Should I add another dose in a day or so if no results? Never had this in the several years we had the pool.

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How To Clean Black Algae After Gunite Pool Is Drained

by Bond
(Tn. 37757)

Though the water was clear our gunite pool developed black algae. We drained the pool completely. How do you clean it and after cleaning will painting the surface help to keep the algae from forming again? What is the best, long lasting paint to buy?

Thanks for the question. Black algae is unlike other forms of algae (green/yellow/brown, etc...) The reason is black algae has "tentacles" and digs into the plaster. Basically, once your pool has it you can only try to contain it and lessen the effects. For your issue, you can treat it the same as you would if the pool was filled, for the most part.

First is to scrape the heads off with a putty knife then rub the affected areas with a chlorine tab. You must grind the chlorine into the algae and the pool surface. You can use a product called Algae-Gon Tab Holder. Once this is finished you'll need to fill the pool, balance the chemicals, then maintain a high FC of 12 - 14ppm for a few days. You can also get some granular Trichlor and put a thin layer on flat affected areas such as the steps and floor. It won't work for the walls.

Maintain a weekly chlorine level of 3 - 5ppm and keep the pH down to 7.2 - 7.4. FC works better at a lower pH.

If you feel your situation is more complex than an email, I do phone and/or SKYPE consultations. It makes things go much faster and many people have found it extremely beneficial, saving them time and money in the long run. All your questions will be answered. I have no chemicals to sell you so you know I'm not bias. If you purchase a personal phone consult you'll get all 3 eBooks for free.

Pool Consultation

Clear Blue Pool eBook

How To Clear Up A Green Pool eBook

Swimming Pool Resources


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