Of the three most common types of in-ground swimming pools (concrete, vinyl liner, and fiberglass), fiberglass pools are sold as having the least maintenance and risk of problems. For the most part, that's true. But as we will learn in this article, water chemistry still matters in fiberglass pools. Especially calcium hardness and overall LSI balance.
Note: There are several reputable fiberglass pool manufacturers in North America. The websites and videos we link to are not brand preferences. They were selected based on the quality of the information and visuals. This article is meant to be brand-neutral.
Covered in this article:
- What is fiberglass?
- Fiberglass pool pros and cons
- Sharp Impacts
- Chemical degradation
- Fiberglass pool chemistry
There are three primary types of swimming pools used in the United States:
- in-ground concrete (or gunite) pools, which have cementitious finishes (plaster, quartz, or pebble),
- in-ground (or above-ground) vinyl liner pools, which use a metal or plastic frame that holds a water-tight vinyl membrane to line the inside of the vessel, and
- in-ground fiberglass pools, which are pre-fabricated monolithic fiberglass vessels, protected by a gel coat finish.
This article focuses solely on fiberglass pools. Fiberglass pools are under-represented in pool chemistry conversations, and they are growing in popularity.
Let's begin by defining fiberglass pools as prefabricated fiberglass pools made in factories. These pools are then delivered in one piece to the job site and lowered into the ground using a crane or excavator of some kind.
The fiberglass is generally about 3/8-inch thick (or thicker) and coated with a smooth gel coat finish. This is different from in-ground concrete pools that have been coated with a fiberglass interior during a renovation. Those types of pools exist too, and the same chemistry considerations exist, but structurally they are quite different than true factory-made fiberglass pools.
What is fiberglass?
Fiberglass is a type of plastic material that is reinforced by glass fibers and resins.1 The result is an incredibly strong strength-to-weight ratio, which is why fiberglass is used across many manufacturing industries (think cars, boats, furniture, and more). Fiberglass is waterproof and durable, largely thanks to its outermost layer: the gel coat.
The fiberglass gel coat is an epoxy resin applied to fiberglass material to give it a smooth, glass-like finish. While durable, it is not invincible. Over time, gel coats can deteriorate, as we will discuss in the chemistry section of this article.
Fiberglass pool pros and cons
Fiberglass pools are pre-fabricated in factories, where conditions are ideal for material strength and consistency. Quality control in a factory is usually superior and more consistent than construction done in the field. The fact that fiberglass pools are monolithic (meaning they are made in one piece) is a major advantage. Provided the hole in the ground and plumbing are installed correctly, there are fewer variables to manage on a fiberglass pool installation.
There are also a wide array of color options for fiberglass because gel coats can be just about any color imaginable. We have seen some fiberglass pools that are absolutely beautiful, and until you get right up close to them, you cannot even tell they are fiberglass. They often look like concrete pools with high-end finishes in them.
Fewer project variables translate into faster installation compared to in-ground concrete pools. From what we have seen over the years, this means fiberglass pools can be less expensive than concrete pools. This is not certain for every pool, of course, because we have seen some elaborate fiberglass pools with intricate decking and outdoor features. Conversely, we have seen many bare-bones concrete pools too. But in general, lowering a one-piece fiberglass pool with a crane is going to be a simpler project than an in-ground concrete pool.
Another advantage is the smooth finish, which is not going to be subject to issues like calcite crystals or calcium nodules.2 That being said, the finish can become rough because it is not immune from prolonged aggressive water or issues like calcium sulfate scale crystals.
In terms of cost, fiberglass pools are usually between concrete and vinyl liner pools. But the options for fiberglass pools are more limited. Design options are limited because a fiberglass pool needs to be made in one single piece, and it needs to be transported to the job site on public roads. So there are some obvious size limitations for fiberglass pools.
There are also hydraulic limitations because most fiberglass pools do not have main drains or floor returns. Usually, all plumbing in and out of the pool is on the walls. This is not necessarily a big deal during normal operation, assuming the pool has a low intake to circulate water from the bottom of the pool...but that's something the manufacturer and installer need to decide to do. If you are considering a fiberglass pool, be sure the company has low suction intakes with VGB-compliant drain covers for optimal circulation.
Now let's look at two structural considerations and the water chemistry of fiberglass pools.
If improper backfill materials are used (like sand), groundwater can percolate in and shift the foundation behind (and beneath) the fiberglass pool. This can lead to settling around the pool. A pool full of water is very heavy, and if the ground beneath it settles or shifts, the plumbing pipes will likely move with it. Plumbing connection failures are one of the main causes of leaks in fiberglass pools. It can be prevented via proper installation and backfilling practices.
To expand on that, let's introduce a concept called flexion.
Visualize ground pressure from outside of the fiberglass pool, pushing inward on the pool. This pressure can force the fiberglass shell to bend, and that flexion will move other parts of the shell to compensate. Since it's one monolithic fiberglass shell, significant-enough pressure from any angle can move the entire shell. In a simplified and exaggerated analogy, think about twisting and bending a plastic ice tray to release ice cubes made overnight in the freezer. That's the same basic idea.
This video from River Pools & Spas explains flexion issues pretty well:
Even if such flexion does not break the fiberglass itself, spider cracking in the gel coat is a common symptom that weakens the finish enough to lead to more problems in the future. Bottom line: flexion is a problem we do NOT want to encounter in a fiberglass pool, so hire a reputable company to install the pool correctly the first time.
The other structural weakness of fiberglass pools is that they do not hold up well against sharp impacts. Sure, this isn't very common, but imagine a heavy piece of pool furniture being thrown into the pool from a hurricane or from vandalism. If an object is heavy and rigid enough, it can crack–and even puncture–a fiberglass pool. One case we heard of was a heavy grill that somehow fell into the pool and smashed into the sun shelf. The pool was ruined.
But it doesn't have to be something so dramatic as a heavy grill. And it doesn't even need to be a puncture to cause a problem. Fiberglass can be scratched pretty easily. Think of metal feet on lounge chairs that might be on a sun shelf or rocks that get into the pool. These can easily scrape fiberglass finishes and leave permanent scars, just like scratching the paint off a car.
The main chemical weakness is the fact that fiberglass gel coats are not immune to low-LSI water damage, chlorine oxidation, and sunlight degradation. In fact, they are quite susceptible to all three. While we cannot do much about sunlight exposure, we can manage water chemistry, and we will elaborate on these issues in a moment.
Fiberglass pool chemistry
Traditional range chemistry (almost)
While manufacturers might differ slightly in their water chemistry recommendations, most include some form of traditional pool industry range chemistry parameters. And just like vinyl liner pools, fiberglass pools often have different recommended chemistry parameters than in-ground concrete pools.
But why? Water is water...regardless of the vessel that holds it. We at Orenda prioritize the water's need for balance, which is why we focus on the LSI first, and range chemistry second. In our opinion and experience, chemistry ranges should be considered within the context of the LSI.
The one parameter that surprises us the most is when fiberglass pool owners are instructed to keep calcium hardness below 200 ppm. The argument we hear most often is that because fiberglass and vinyl pools don't have readily available calcium, calcium hardness is less important in such pools.4
We strongly disagree. If anything, LSI balance is more important in fiberglass pools! Water will stop at nothing to find the LSI balance it craves.
Low calcium hardness virtually guarantees aggressive water for most of the year; water that will degrade the gel coat of the pool and lead to problems like chalking.
The main concern is aggressive (low-LSI) water, which attacks and weakens the gel coat. Once the gel coat is compromised, chlorinated water can oxidize the material from within, which turns the surface a hazy white color. We call this fiberglass chalking. It's the byproduct of degradation and oxidation, and the damage is permanent.
Related: What is fiberglass "chalking"?
The gel coat can become compromised over time from prolonged exposure to aggressive (low-LSI below -0.30) water. Unlike in a concrete pool with a cement-based finish, there is no calcium for hungry water to dissolve...but that does not stop aggressive water from looking for calcium. In fact, we would argue that LSI is as important (if not more important) in fiberglass pools because of this.
In a concrete pool, water has plenty of calcium around it to balance itself.3 In a fiberglass pool, water has no means of correcting its own LSI, so if it's hungry enough for calcium, it will destroy a fiberglass gel coat in search of calcium...and it will continue to destroy until the LSI gets balanced.
In other words, because there is no calcium to be had, water stays aggressive (and destructive) longer.
If a fiberglass pool is turning white, it's probably chalking, not carbonate scale. One way to tell is to know the water chemistry history and what the LSI has been. Another is to put diluted acid on a rag and wipe the affected areas (wearing safety gloves and glasses, of course). If the white discolorations are actually carbonate scale, they should fizz and bubble as the acid dissolves the calcium carbonate. If nothing happens, it's probably chalking.
How to fix oxidized fiberglass (chalking)
Fixing fiberglass chalking requires removing the layers of oxidized gel coat and reapplying a new gel coat. It involves a multi-day process where the pool needs to be drained, and it can be costly. Here's a good video we found online demonstrating the process:
While an oxidized gel coat on a boat or car can be buffed and waxed, fiberglass pools cannot. This is because fiberglass wax and sealant products are organic and petroleum-based. CV-600 enzymes would devour such products, and chlorine would oxidize them too.
An alternative to applying a new gel coat would be to install an ecofinish, demonstrated here:
Proactive, LSI-focused chemistry
We prefer a more proactive approach.
Since water cares only about its balance–which we measure using the LSI–it makes logical sense to focus on maintaining LSI balance. It's our first pillar of proactive pool care for a reason. LSI balance is critical in every swimming pool, especially fiberglass.4 When the water is balanced, and the pH is contained within that LSI balance, our focus can shift toward water quality instead of fighting water balance issues.
Yes, this means planning ahead. Yes, this often means having more calcium hardness than recommended. Yes, this means allowing the pH to stay at its ceiling during the winter months (because it's going to anyway...it's physics). So take all six factors of the LSI into account, and any fiberglass pool should stay beautiful for a very long time.
How to maintain a fiberglass pool year-round
Remember, there are two disciplines of pool chemistry: water quality and water balance. From a water quality perspective, there are no significant differences between fiberglass and other types of pools. And like any other pool, the focus of water balance is to keep the water LSI balanced year-round.
To maintain LSI balance year-round, we must account for water temperature! If the pool is in a location that experiences different seasons and cold temperatures, water temperature drives our water balance strategy at different times of the year.
We recommend maintaining the pool just like any other type of pool. For water quality, avoid over-chlorination and algaecides. For water balance, the Orenda Calculator™ will show you your options, and you can take Orenda Academy for more information too.
Out-of-season and winterization
Most fiberglass damage occurs during the winter. In fact, most damage to any type of pool occurs during the winter too, because the water is cold and, therefore, the water is more aggressive. Winterization, on the water balance front, is as simple as preparing the water for the cold months ahead. On the water quality front, remove contaminants before closing the pool so it's easier to open in the spring. And be mindful of the type of pool cover used because it impacts the chemistry needed for the winter.
Structurally speaking, fiberglass pools are different than in-ground concrete pools and any type of vinyl liner pools. In many ways, they are lower maintenance and provide great value for the dollars spent. That being said, fiberglass pools are not invincible, so proper installation and maintenance are required.
The single biggest chemistry issue we hear about is chalking, which is the result of low-LSI water attacking the gel coat, which allows for chlorine to oxidize the material within, turning it white. This is a preventable problem by simply maintaining LSI balance year-round. We hope this article is helpful and debunks the myth that fiberglass pools need less calcium compared to other types of pools. Water is still water, and it still craves balance.
1 One of the best resources for fiberglass information is from Fibreglass Direct website.
2 Calcite crystals and calcium nodules are the result of calcium hydroxide coming out of a cementitious finish, either from being pulled out (crystals) or pushed out (nodules). But neither of these can form in fiberglass pools because there is no calcium in the material. There is an exception to this, and it is worth an explanation. There have been rare cases of calcium nodules forming in fiberglass pools, but these were not pre-fabricated fiberglass pools. These nodules formed in concrete pools that had been renovated with a fiberglass finish on top of the old plaster finish of an in-ground concrete pool.
3 It should go without saying that water balancing itself at the expense of cement pool finishes is still undesirable. This is called etching, and it is also permanent damage. The difference between cementitious finishes and fiberglass gel coats, in this regard, are two-fold. 1) when water dissolves calcium out of cement, it balances itself and stops attacking the surface (at least for a while, until someone re-adjusts the water and over-corrects with acid, let's say), and 2) there are many treatments that can be done to restore the look of a cementitious pool finish. Fiberglass gel coat damage is much harder to deal with, because it requires draining and re-applying a new gelcoat. Sure, there are plenty of other differences between cement and fiberglass; we're just saying that from a water chemistry standpoint, low-LSI aggressive water in a fiberglass pool tends to stay aggressive much longer and continues to attack the surface because there is no calcium to be had.
4 Cement-based finishes are rich in calcium. Vinyl liners also (surprisingly) contain calcium carbonate within them. But fiberglass has no calcium for water to dissolve and correct its own LSI balance. And while fiberglass may be temporarily resilient to aggressive water, rest assured this is only temporary. There comes a point when the gel coat is compromised enough that the gel coat itself begins to be oxidized and turn white. This is what we call "fiberglass chalking".