Pool Liner Stains

Is it ok to put chlorine tablets in my skimmer instead of using a chlorine feeder?

The answer is yes, many people do just that, but you can get in trouble if you don’t run your pump runs 24 hours a day.  We would not recommend putting tablets in your skimmer if you operate your pool on a timer and you can also get in trouble if the power goes off.

The chlorine tablets are concentrated and they have a very low ph.  When your pump is running, the water going through your skimmer is constantly eroding the tablet, adding chlorine to the water which quickly makes its way through your filter system and into your pool.

If the pump goes off, now that tablet is sitting in just a few gallons of water and the chlorine level rises and ph drops. Now you have a skimmer full of highly chlorinated/acidic water and when it sloshes out of the skimmer and back into the pool with the regular movement of the water in your pool, that caustic water runs right down the face of the wall and can burn your liner as shown in the photos below.  This will damage and permanently discolor your liner. Another thing to consider is whether or not you’re heating the pool. The highly-chlorinated water can lead to copper corrosion on the heater unit.


What are the black stains on the floor of my liner and how do I get rid of them?

If you have black or grey cloud-like stains on the floor of your in-ground vinyl liner, they are most likely caused by bacteria that is living in the ground under your pool.  Bacteria sometimes show up years after a pool has been installed.  We think the bacteria can be washed into your pool area by flooding rains or when a septic tank overflows.

There is little proven research on this topic but we understand the bacteria actually gets sunlight through the water, eats on some food source in the ground and then secretes a dye-like material that migrates through the vinyl and shows up as a stain on the floor of your pool. Often these spots appear grey, greyish green or light black in color. Once the spots appear, the damage to your liner is done and it’s irreversible.  Algaecides and other chemicals used in the pool will have no effect on these stains since they can’t get to the source of the problem which is the bacteria in the soil.

So you live with stains as long as you can and then, when it’s time to change your liner, it’s also time to try and get rid of the bacteria to avoid a repeat problem with staining. No one knows precisely how to solve this problem but there are four methods that we understand may be effective.  You may want to choose one or even try all four.

  1. The simplest way to attempt to kill the bacteria is to have your contractor spray the floor of the pool with liquid chlorine before installing your new liner. Chlorine bleach works fine and they will mist the floor of the pool using a garden sprayer three or four times.
  2. A second possible solution is to change the pH of the soil under and around the pool.  This is done with a chemical called Copperas Iron Sulfate (FeSO4) which is available at tree nurseries or agricultural supply houses. The chemical is sprinkled on the ground around the pool and on the floor of the pool and then the ground is saturated with water for two to three days. The idea is to get the powder deep into the ground so it can change the pH of the soil and kill the source of the bacteria.
  3.  Since bacteria need sunlight to grow, a third option is to eliminate the light source by putting a black sheet of plastic under the liner. This is not advisable if your yard has a high water table as rising water may float the liner and ball up the plastic underneath causing an even bigger problem than the stain.
  4. Finally, some companies are selling a roll-on epoxy-like stain barrier paint to cover the floor and walls of your pool.  It’s like a liquid plastic that sets up hard to form a membrane intended to block out the light and prevent stains from getting to your liner. It is the most expensive of the options but some people claim it really works.

Discuss these options with your local pool professional and depend on them to suggest which method they have tried and found to be most effective.

Moldy Pool

My vinyl liner pool is dirty. Can I drain the pool to make it easier to clean?

As a general rule, once a liner is installed, the pool should never be drained.  You may get away with it in the first few years but after that, the liner starts to lose its elasticity and you run the risk on the liner ripping or tearing when you try to refill the pool. This is the same problem you encounter if the liner has floated and needs to be reseated.  Our advice would be to always clean a pool while still full of water.  Fish out the big debris and then let the filter and the chemicals do their job.  If it’s just dirty, the chlorine will burn up the organic stains, given time.  If you have metallic stains, click here to see our previous post on how to identify and address those.



How can I prevent my pool liner from having discoloration, deterioration and “dry rot” above the water line?

We recently had a homeowner ask a question in response to our blog post, What are the causes of discoloration, deterioration and “dry rot” above the water line on a pool liner?

My pool is doing all three of these and it is only six years old. How can I prevent this from happening again?

As our blog article states:

There are three main contributors to this problem: chemical attack, high temperatures and UV rays.  However, the UV resistant characteristics of pool vinyl are excellent and the UV rays themselves do not present a significant problem.

Clean your pool often by taking a soft cloth and rinse the contaminates from the vinyl using the pool water.  Substances such as body oil, sun tan lotion, baby oil, etc., will collect at the water line.  These substances, when exposed to the sun and the high temperate that can be found just above the water lines, will oftentimes turn brown and can be very difficult to remove from the vinyl.

We should have added that chemical attack means high chlorine and/or a low ph.  The two things you can do here are to not over chlorinate and to keep your ph at 7.2 or higher.  These are the two most critical things you can do to prevent dry rot.

The other issue is high temperatures.  Your pool vinyl is not intended for use over 80 degrees.  On the hottest summer days, you can direct your return nozzles up so that the water shoots up out of the pool.  This will cool the water overnight.  If you have a heater, don’t set it too high.

I would also add that covering your pool during the off season can significantly increase the life of the liner since it’s out of the sun and without chlorine for much of the year.  I think this is why liners always last longer in NY than they do in Texas.

I would also add that this homeowner’s liner made it six years and the average life is seven.  It failed a little earlier than expected, but not by that much.

Hope this helps!

What causes staining or discoloration of the vinyl below the water line?

While all stains are either mineral or organic, the most common cause of staining and discoloration of your liner below the water line is secretions by micro-organisms.  As these micro-organisms feed, they secrete dyes, which can be one of many colors that stain the vinyl.  Although these stains are unsightly, they in no way degrade the performance of the vinyl.  These dyes are compatible with the plasticizers in the vinyl, causing the stains to go all the way through the sheet.  There is no proven method for removing these stains.

There is a common misconception that the microbial resistance additives used in pool liners will kill the micro-organisms in the area adjacent to the liner.  Many people believe that there is a “protective zone” near the liner that will not support life, but this is not the case.  The additive in the vinyl prevents the vinyl from supporting life, but in no way does it prevent life in areas adjacent to the liner.  Extreme care must be taken during installation to insure that there is nothing behind the liner that may become a food source for these organisms.

There have been cases of stains forming in pools soon after the installation of a replacement liner when there was never a problem with the original liner.  Although there is no way that we can say for sure what has happened behind that new liner, it is believed that when the environment behind the liner is exposed to light and oxygen, a “rebirth” of micro-organisms takes place.  If the bottom and sidewalls of the pool are not properly treated, there is a chance that problems may arise.

There can also be changes in the ground water that introduce organisms into an area that had not been previously exposed.  Extended periods of heavy rain will often cause significant changes in the microbiology of the ground water.  Whenever there is a change in the environment around your pool, there is an opportunity for micro-organisms which hitherto were not present to move into the ground water, thereby creating the possibility of staining.

*The information provided comes from a liner manufacturer.

I have a stain on my vinyl liner. How can I tell if it is from algae or minerals?

This is a tough question. Below is a suggestion from one of the product engineers at the plant that makes the actual vinyl sheet.

The following approach could be used to narrow down the possibilities; however, there is risk of damage to the print pattern and liner material in doing so.

First, try direct contact on a small spot with some pH down in a sock to see if there is any change in the stain. If there is no change, the same approach can be tried with a neutral or high pH sanitizer (i.e. dichlor or calcium hypochorite).

If the pH treatment alone reduces the stain, it is likely a mineral stain. If the chlorine application reduces the stain, it is a microbiological stain, such as an algae.

If the stain is from algae growth on the back side of the liner, any success on removing the stain will likely only be temporary.

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