Helpful Tips On How To Remove Stains From A Fiberglass Pool

Fiberglass pools tend to be easier and less expensive to own than other types of pools. They require fewer chemicals and less electricity to correctly maintain and seldom suffer from stains when properly managed. But that doesn’t make a fiberglass pool immune to staining.

Wipe It Off

Pool stains need to be cleaned as soon as possible since they become more difficult to remove the longer they are left unaddressed.

The first step is to try to wipe or scrub the stain/discoloration off the fiberglass. Be careful not to use abrasive scrubs, hard brushes, or harsh chemicals… use a soft sponge or cloth, and try to only use products specifically intended for use on fiberglass pools. Rough materials and harsh chemical abrasives might inadvertently damage the gelcoat of the fiberglass pool shell surface, compounding the problem.

If the stain or discoloration cannot be wiped away or removed by simple scrubbing, you will need to further determine the cause and type of the stain. Please contact your local certified pool maintenance company.

Is the stain organic, or inorganic?

If you are unsure of the nature of the staining, you can use a commercial stain identifying kit (chemical testing agents you add to the pool water) to determine what sort of stain you’re dealing with. But in many cases, the cause of the stain will be easy to visually detect. Before using any chemicals please contact your local certified pool maintenance company for more information.

Common Sources of Organic Fiberglass Pool Stains

  • Leaves, twigs, brush
  • Fruit or berries from overhanging or adjacent plants or trees
  • Dirt
  • Algae: though this isn’t technically a stain, black algae can appear as black spots growing on the pool shell.

Common Sources of Inorganic Fiberglass Pool Stains


Metal staining was formerly a real problem on older models of white fiberglass pools, though it’s no longer as common on modern pool shells.

Identify the Type of Metal-Based Stain by the Color:

  • Iron (rust - orange, brown, or tan)
  • Copper (verdigris - green or blue)
  • Manganese (brown, black, or purple)
  • Cobalt stains (black spots on the pool shell surface)

    Though “cobalting” is technically not a stain, it is a condition that occurs when pool water penetrates pinhole gaps in the gelcoat and chemically reacts with the resin beneath by osmosis, creating crystallized areas of black discoloration on the fiberglass pool shell.



"Calcium scaling" also isn’t technically staining. It’s a condition caused by calcium crystals that accumulate on the surface of the pool. Though not itself a staining agent, it can still cause discoloration and damage the fiberglass pool shell if allowed to build up on pool surfaces.

Removing Fiberglass Pool Stains

Organic stains can often be removed with simple contact cleaning, by brushing.

If a brush doesn’t completely eliminate the stain on the first try, be aware that really tough stains may require multiple contact cleaning sessions. The passage of time (plus consistent exposure to UV sunlight) will aid you in encouraging stubborn stains to fade naturally.

Repeat the scrubbing process regularly as necessary. Most organic stains should fade and disappear when treated. If they don’t, consider having the stain professionally treated by a certified pool maintenance company or try contacting them first if uncertain.

Metal Stains


Often you can eliminate most rust stains by scrubbing the area with a chlorine tablet (don’t rub too hard, lest you damage the fiberglass surface). Another option is to apply a Vitamin C tablet directly to the stain itself, which can act to chemically dissolve the stain. Use a soft-bristled brush to remove any excess rust.

If your fiberglass pool has been sufficiently contaminated by rust that you have multiple rust spots and extreme staining on your pool shell, you can fight back by running ascorbic acid (Vitamin C) powder through your pool filter. Be sure to follow your pool manufacturer's specifications for the best results. And again, if the staining is too great for you to remedy, a call to a professional fiberglass pool maintenance company may be in order.


One way to remove copper-based, turquoise-colored verdigris stains is by scrubbing them with a mild commercial tile cleaner. Afterward, you can follow up by treating your pool water with chelating or sequestering pool chemicals (specifically intended for copper stain removal). Always follow the chemical manufacturer's instructions regarding amounts and the proper dilution level based on the water content of your pool. These products will cause any copper still in the pool water to form clumps that can be removed by cleaning or filtering the water.

Other Metals

Most non-ferrous metal stains can be removed by scrubbing them with a mild commercial tile cleaner.

Calcium scaling

You can remove calcium carbonate by adding a fiberglass pool safe scaling treatment to the water.

Calcium silicate build-up almost always requires a professional fiberglass pool safe scaling treatment. Consult the professionals at a certified pool maintenance company for a list of options.

Ring Around the Pool

An ugly discoloration (like that of a bathtub ring) can appear around your pool’s waterline, caused by an accumulation of suntan creams, body oils, and excess metals from your pool maintenance chemicals.

Remove oily stains from the waterline by scrubbing with a soft cloth or sponge. You can prevent oily buildup by using an enzyme in the water. You’ll have to filter the water and restabilize your pool’s chlorine and PH levels afterward. NOTE: it’s always a good idea to consult a fiberglass pool maintenance professional before using such additives.

Avoiding Fiberglass Pool Stains In The Future

The best offense is a good defense: prevention is the best way to combat fiberglass pool stains. Since these actions are a common part of standard pool maintenance it’s likely that you are already performing them regularly: maintain your water chemistry in the ideal range, add water treatments and preventatives whenever necessary, and regularly check your pool's pH to keep it at ideal levels. Clean your pool regularly by sifting or vacuuming any foreign matter (dirt, leaves, insects) to prevent waterline staining, and carefully removing any foreign metal objects from your pool before they have a chance to rust or corrode from water exposure. Be sure to run the filter system throughout pollen season and while deciduous trees are shedding their leaves, to circulate the water and prevent foreign matter from settling onto the fiberglass surfaces of the pool shell long enough to stain it.

If you are forced to deal with the unsightly staining of your fiberglass pool, do not despair. There are many ways to eradicate such stains, as we have outlined here. If you're unable or unwilling to try to remove the stain yourself, consider contacting a professional company that specializes in cleaning and maintaining fiberglass swimming pools.