Fiberglass Pool Water Chemistry

WHAT REALLY PROTECTS A FIBERGLASS POOL FINISH?

Your fiberglass pool’s finish—its smooth, nonporous gelcoat—is one of the most durable pool surfaces available, but it’s not immune to damage from poor water chemistry. Traditional pool chemistry standards were built for concrete pools, where calcium plays a critical role in surface integrity. Fiberglass is different.

Fiberglass pools don’t require calcium to maintain strength or prevent corrosion. In fact, excess calcium in combination with high pH can do more harm than good, leading to chalking, discoloration, and irreversible surface damage.

The best way to protect your fiberglass pool is to stop following outdated “one-size-fits-all” guidelines and start following fiberglass-specific recommendations. Don’t be standard. Your pool isn’t.

What truly protects your fiberglass finish is consistency in the following fundamentals:

• Tight pH Control: Maintain between 7.2–7.4 to prevent surface stress and maximize sanitizer efficiency.

• Low Calcium Hardness: Keep below 120 ppm to avoid scaling and calcium-related discoloration. If source water is above 120 ppm, you must regularly use a stain/metal and scale control product.

• Moderate Free Chlorine: Maintain 1–3 ppm and avoid over-shocking or using calcium-based chlorine products like Calcium Hypochlorite. Liquid chlorine is a great option.

• Regular Use of Stain & Scale Control: Chelating agents like Orenda SC-1000 or Pool Stain Remover’s Stain & Scale Eliminator bind to calcium and effectively shield your fiberglass pool finish.

• Total Alkalinity: Maintain between 80–120 ppm to help buffer pH changes.

• Cyanuric Acid (CYA): Keep between 30–50 ppm to protect chlorine from UV degradation without over-stabilizing.

• Metals: Keep metal levels at 0 ppm to prevent staining and discoloration.

• LSI Range: Use as a reference only. If used, target -0.30 to +0.30, understanding that a slightly negative LSI is better tolerated by fiberglass than a positive one. Under no conditions should LSI go above +.30.

The goal is balanced, gentle water that’s easy on your surface— and safe for your swimmers.

WHY HIGH CALCIUM AND HIGH pH DON'T MIX

One of the most common—and preventable—causes of fiberglass pool discoloration is the combination of elevated pH and high calcium hardness. Together, they can discolor the fiberglass pool finish. This cosmetic damage is often permanent and significantly diminishes the look of your gelcoat surface – this type of damage is less noticeable when the surface is wet and more noticeable when it is dry.

To avoid this, do not use calcium-based products (like Calcium Hypochlorite or Calcium Increasers) and use a stain and scale control product as part of your regular care routine.

WHAT ABOUT LSI?

LSI can be a useful reference point, but it should not be the cornerstone of fiberglass pool care. The LSI was originally designed for concrete pool structures that require calcium to prevent etching. Fiberglass pools don’t operate under the same rules.

If you choose to use the Orenda app or another LSI calculator, treat it as a guide— not a strict rulebook. Aim to keep your pH, calcium, and sanitizer levels within the recommended ranges. A slightly negative LSI is generally safe and even preferable for fiberglass.

ROUTINE MAINTENANCE MAKES A MAJOR DIFFERENCE

Regular maintenance, including water chemistry, appropriate cleaning measures, and equipment upkeep, is essential to ensure the quality and longevity of your fiberglass pool.

Without proper maintenance, various issues could arise and potentially lead to a need for costly, extensive repairs.

Poor water care can cosmetically damage the pool surface through fading, etching, or chalking and contribute to an increased risk of bacteria and other contaminants. Poor water quality can also affect swimmers’ comfort and damage pool surfaces or equipment. Neglecting regular cleaning and routine maintenance of the pool’s pumps and other equipment can further exacerbate water quality issues

At a minimum, conduct weekly tests and make the necessary adjustments to ensure you’re following the ranges and recommendations for fiberglass-specific care guidelines. For optimal water quality, sanitize the water by maintaining free chlorine levels between 1 and 3 ppm.

Regularly vacuum the pool and wipe down the water line as needed to keep it looking pristine. Stay on top of water circulation maintenance by keeping the pool’s filter media clean and frequently emptying the skimmer and pump baskets.

A well-maintained fiberglass pool is a visually appealing backyard entertainment and leisure centerpiece. Regular maintenance ensures the water is inviting and the pool area remains a source of pride and enjoyment.

MAINTAINING YOUR WATER CHEMISTRY IN THE OFF-SEASON

Maintaining proper water chemistry during the off-season is crucial for the health and longevity of your fiberglass pool, even when it’s not in regular use. The colder months can bring unique challenges, as the water chemistry can become more volatile with temperature fluctuations. Here’s a detailed guide to keeping your pool’s water chemistry maintained in the off-season:

Balance Before Closing: Before covering your pool, test and adjust pH (7.2–7.4), calcium hardness (under 120 ppm),  chlorine (1 -3 ppm), and alkalinity (80–120 ppm). No additional shock treatment is needed prior to closing.

Use a Stain & Scale Control Product: Add a chelating agent to manage metals and calcium. This is particularly important during the winter, when water becomes more reactive.

Skip the LSI Chase: Use LSI only as a general reference, not a goal. Focus on range-based chemistry. Cold water naturally drops LSI, which is okay for fiberglass.

Monthly Checks: When possible without damaging your pool cover, it’s important to continue monitoring your pool water chemistry throughout the winter. Weather conditions can affect the water balance, so monthly checks are recommended. Test the water and adjust the chemistry as needed during these checks to maintain the LSI balance. This proactive approach can prevent imbalances that might otherwise go unnoticed until they’ve caused significant issues.

Proper Closing Practices: When closing your pool, ensure it’s clean and debris-free. Lowering the water level according to the manufacturer’s or pool professional’s recommendations, properly winterizing the equipment, and securing a durable pool cover are all steps that will help maintain the water chemistry balance and protect the pool over the winter.

Seek Professional Guidance: If you’re unsure about any steps in the winterizing process or encounter issues while balancing the water, don’t hesitate to consult a pool professional. They can provide valuable insights and assistance to protect your pool during the off-season