The Easy Way to Keep Your Pool Water Balanced with LSI

If you just wrapped up a pool renovation (or even if you didn’t), LSI should be on your radar. It’s the secret to keeping your plaster fresh and tile in tip-top shape. Maintenance is like an insurance policy for your renovation, and LSI is one of the most important aspects of pool care.

LSI (short for Langelier Saturation Index) helps keep your water balanced so it doesn’t eat away at your surfaces or leave behind that annoying, chalky, white buildup. Basically, it keeps your pool healthy and your renovation protected. When your water’s just right—not too corrosive, not too scale-forming—your surfaces and equipment stay in great shape.

And a well-maintained pool isn’t just beautiful, it boosts curb appeal and resale value. (Yay!) Neglect, on the other hand, leads to early wear and tear, costly repairs, and a dip in property value.

Here’s what you need to know to keep your pool looking as fresh as the day the plaster dried.

What is LSI?

LSI is the scientific formula used to determine if your pool water is balanced, corrosive, or scale-forming. 

It’s the science behind healthy pool water, and a must-know if you’ve invested in new plaster or a new pool finish. 

LSI takes into account:

• pH
• Alkalinity
• Calcium hardness
• Water temperature
• Total dissolved solids (TDS)

Low LSI (negative value) means your water is too corrosive. Water with low LSI will start pulling calcium from plaster, tile, grout, and even metal parts. That leads to rough surfaces, pitting, calcite crystals and expensive repairs. 

High LSI (positive value) means your water is too scale-forming. It’s what causes that chalky white buildup, known as calcium scale, on tile, plaster, and equipment.  

By keeping each of these balanced, LSI helps protect your pool’s surfaces and equipment while extending the life of your pool’s surface.

How LSI Affects Plaster and Fiberglass Pools

Not all pools react the same way when your water chemistry is out of wack. 

If You’ve Got a Plaster Pool…

Plaster is made with calcium, and your pool water knows it. When your LSI is too low (which means your water is too corrosive), it actually starts pulling calcium out of the plaster. That’s when you get etching, pitting, and tough spots that feel like sandpaper under your feet. 

When your LSI gets too high your water tries to dump extra calcium onto the surface. And that’s when you see that chalky white buildup that’s annoying and tough to clean. 

Plaster is beautiful, but it requires balanced water to stay smooth, strong, and looking good. 

Fiberglass Is More Chill-but Still Needs Attention 

Fiberglass (and gelcoat finishes like INTER-GLASS®) are a bit tougher when it comes to chemical drama. They don’t have calcium to give up, so they won’t etch like plaster does. But LSI still matters.

If the LSI dips too low, you could end up with stubborn stains or a dull, tired-looking surface.

Too high, and you’ll get that same calcium scale that sticks to your finish (and your equipment…yikes).

So while fiberglass isn’t quite as sensitive, keeping the water balanced still makes a big difference in how your pool looks, and how long everything lasts.

Maintaining LSI Year Round

Unfortunately, LSI isn’t a set-it-and-forget-it deal. Water chemistry changes with the seasons, especially in colder months. 

When temperatures drop, so does your LSI, making the water more corrosive. While low LSI is always a concern, winter poses unique risks—especially when your pool is covered and out of sight. Cold, aggressive water can pull calcium from the surface, leading to stubborn issues like sharp calcite crystals, winter calcium dust, and ghosting—a whitish mottling throughout the pool. These winter-specific issues are tough to remove, so it’s crucial to stay vigilant even in the off-season.

During the summer months, hot temps can push LSI too high, causing scale buildup. 

The trick to keeping LSI balanced all year long is to test your water regularly, even in the off-season, and stay ahead of those sneaky seasonal shifts.

Want to Dive Deeper and Get More Technical?

To learn more about LSI click here. Hear from pool maintenance expert John Cooper, owner and lead chemist at Pools Scientific, a Kansas City-based swimming pool service and maintenance provider. 

Don’t forget your equipment
Old or faulty filters and chlorinators can mess with your water and damage new plaster.

• Clean filters monthly
• Inspect your chlorinator
• Backwash as needed
• Upgrade to automation if possible

Stay vigilant to protect your investment
Even with automation, regular checkups matter. Keeping your pool balanced year-round protects your renovation, and your investment.

Ready to love your pool again?
Get a free quote from the most trusted name in pool renovation and resurfacing. With decades of experience and stunning results to match, we’ll bring your pool back to life better than ever.