What Can Damage a Fiberglass Pool?


It’s easy to get caught up in the excitement of something new. Whether it’s a car, a boat, or an inground swimming pool, it’s only natural to want to do as much research as possible before signing on the dotted line. You’ll want to know how long it’ll last, right? If you’re currently considering an inground fiberglass pool and trying to cover all your bases, you may have searched for potential problems such as, “What can damage a fiberglass pool?” and “Can you scratch a fiberglass pool?” Perhaps you’ve also looked up fiberglass pool gelcoat problems and fiberglass pool bulging walls. This post covers a few common concerns you may have heard regarding fiberglass pools, how they happen, and if you can prevent them or repair them if they occur.

How Does Fiberglass Pool Chemistry relate to these issues?

A big part of the longevity of your fiberglass pool is the water chemistry. Water chemistry can be tricky for any pool owner, and it’s best left to the professionals that do pool maintenance for a living.However, as a fiberglass pool owner, it’s important to understand water chemistry guidelines so you can take the best preventative care possible of your pool shell. You might have heard someone ask, “why is my fiberglass pool turning white?” It is often because of an imbalance in the water chemistry. A fiberglass pool surface finish is very durable but it is not indestructible. Water balance is vital to ensuring protection for the surface finish of your fiberglass pool.

Remember that even when using a professional to manage your fiberglass pool’s water balance, you will want to take responsibility and know the parameters  of keeping your water balanced. Balanced pool water is neutral, meaning that the water is neither corrosive nor scaling. Bear with us – we’re going to get into a bit of science here.

 A commonly used measure for water balance is the Langelier Saturation Index (LSI). The LSI is a model developed to estimate the diversified state of water by W.F. Langelier, a professor at the University of California in Berkeley. Paraphrasing The Journal of Water Resource and Protection, Vol. 9, No. 2, February 6, 2017, the LSI can be used to control corrosive waters incorporating parameters including acidity, alkalinity, pH, and calcium ions. The proper LSI level is 0.

Calcium levels that are too high can lead to rising pH levels in the water, causing white deposits to appear on the pool’s surface. Often, the manufacturer of your pool shell provides you with an owner’s manual that gives water level recommendations, so we suggest following those. Keep in mind that our recommendations are specifically for fiberglass pool shells. Our owner’s manual recommends your calcium levels remain between 150 and 200 parts per million.

We recommend the chlorine residual in your pool, which is the chlorine leftover once disinfection and oxidation has occurred, should be maintained at one to three parts per million and never exceed five parts per million as this can damage the pool surface.

Per our recommendations, the pH level, which measures the acidity or basicity of water, should be maintained between 7.2 and 7.6.

The total alkalinity measures the amount of bicarbonate material in pool water and indicates the water’s capacity to withstand changes in pH. We recommend it be 80 to 120 parts per million.

Finally, cyanuric acid prevents the decomposition of free chlorine in the pool water. As a stabilizer for chlorine, we recommend your cyanuric acid levels  be at 20 to 50 parts per million. As levels rise above 100 parts per million, chlorine is not as effective at killing bacteria and algae. 

Thanks for sticking with the science there. It’s helpful to understand it to ensure the person maintaining your water balance is doing it correctly. You may also want to familiarize yourself with water test kits and educational booklets such as “Pool & Spa Water Chemistry” and “I Never Liked Chemistry.” 

Other surface issues, like spider cracks in fiberglass pools, are generally surface-level concerns and don’t impact the pool’s structure. These cracks can occur when a large amount of pressure is applied to an area or manufacturing processes render the gelcoat too thick. Scratches can occur on the surface, as well. However, gelcoat in fiberglass pools tends to be tough (but smooth) and resistant to scratches, whether they be from you or your dog’s paws, so hopefully you won’t run into this yourself. 

As previously mentioned, some issues can occur primarily due to improper manufacturing practices, such as osmotic blistering. These tiny bubbles appear on a pool’s surface when water molecules pass through the pool’s gelcoat and meet with the polyester resin layered inside. Though they don’t look very appealing, they also don’t impact the pool’s structure and are purely a cosmetic concern.

Can Issues With Fiberglass Pools Be Fixed or Prevented?

Discoloration/Residue: Regular water testing, proper water chemistry, and certain equipment add-ons can prevent discoloration and deposits on a fiberglass pool’s surface by suspending calcium and reducing the need for high levels of chlorine. Obtaining guidance from a professional is the best way to address these issues, but the homeowner can often perform the correction methods.

Spider Cracks: Fiberglass pool gelcoat repair methods are available to restore the gelcoat after cracking. Your independent pool builder or maintenance company will likely be able to point you in the right direction for who can provide these services.

Pool Wall Bulging: When it comes to fiberglass pool walls bulging: yes, it can happen. But generally, it is due to the pool’s structure or incorrect installation practices. When not properly installed or not durable enough in structure, a fiberglass pool’s walls can bulge due to groundwater pressure combined with the earth’s natural tendency to shift. The best way to prevent this issue is to make sure you go with a manufacturer that accounts for both flexibility and rigidity in their pools’ structures. Some innovations, like the Thursday Pools Geo-Anchoring Pool wall®, provide additional structural support by anchoring the pool to the earth to help prevent bulging or shifting if water must be lowered according to the TP owners manual.


We always applaud those who do their research before splashing into the world of fiberglass pools. We hope this has not only given you a better understanding of what can damage a fiberglass pool, but also made you feel more comfortable moving forward in the pool ownership process. 

If you’d like to get in touch with an independent dealer in your area, click here. Don’t feel quite prepared to take that next step? We encourage you to try out our pool cost calculator for ballpark estimates or our Idea Book for backyard inspiration.

Thursday Pools produces one-piece inground fiberglass swimming pools that come in various shapes and sizes. All Thursday Pools fiberglass pools meet strict ISO 9001 standards for quality construction and materials and ISO 14001 standards for environmental stewardship. Our pools are backed by two industry-leading warranties and are crafted with true passion and teamwork

Get a free estimate on a new fiberglass pool today. With Thursday Pools, your weekend starts early!