If you’re reading this, chances are you’re frustrated by those mysterious white stains in your pool — and you still haven’t found a clear answer. Don’t worry. At Pool Health, we dig a little deeper.
So, how often do you see white marks on your pool? And are you sure they’re scale?
Most pool specialists will quickly blame calcium scale, but the truth is more nuanced. Science doesn’t have every answer yet, but what we do know is this: not every white spot is scale. Sometimes, it’s actually calcium crystals.
What’s the difference?
Take a look at these two photos (scale vs. calcium crystals). Do you notice how they look different? Let’s break it down.
Why LSI matters?
The most important factor in water balance is the Langelier Saturation Index (LSI). This index tells us whether water is aggressive (corrosive) or scale-forming. Think of it as a report card on how your pool water behaves toward surfaces and equipment.
• If LSI is below -0.3: water is corrosive — or “hungry.” It doesn’t have enough calcium, so it starts dissolving it from plaster, grout, stone, tile, liner, fiberglass. That’s how you get etching and calcium crystals.
• If LSI is above +0.3: water is scale-forming. It has more calcium carbonate than it can hold, so it glues it on the hottest surfaces in the form of scale.
• If LSI is around 0: water is perfectly balanced. That’s the sweet spot you want.
How does it connect to white stains?
• Low LSI (hungry water): The water steals calcium from your pool surfaces to reach equilibrium → calcium crystals form.
• High LSI (overloaded water): The water forces out extra calcium carbonate → scale forms.
In short:
• Crystals = water pulling calcium in.
• Scale = water pushing calcium out.
What science says?
The latest detailed research on this topic comes from Orenda Technologies (2017–2018). Their work clarified how water chemistry, especially the LSI, directly determines whether you end up with scale or crystals.
White stains don’t always mean the same thing. If you understand LSI, you understand your pool.
Balanced water isn’t just about clear water — it’s about protecting your pool from being eaten away or covered in crusty scale.