A white, crusty line at the waterline is one of the most visible pool problems, and one of the most stubborn. Scrubbing it off without fixing the water means it comes back.
The short version: scale forms when the water is scale-forming: Taylor Technologies links an LSI of +0.50 or higher to cloudy water and scale on surfaces, and TroubleFreePool says calcium above 650 ppm can cause scale. Fix the balance first, then remove the deposits with a method that is safe for your tile.
Part of: Pool Water Balance: Calcium, Scale and the LSI, our overview of water balance.
Why it forms
- Scale-forming water. High pH, high alkalinity, high calcium and warm water all push the saturation index up (our explanation of the LSI formula). Above +0.50, Taylor says, expect scale.
- High calcium. TroubleFreePool: above 650 ppm, scale on pool walls and in salt cells.
- The waterline itself. Water evaporates at the tile line and leaves its dissolved minerals behind (our explanation).
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Test kit A test kit that reads calcium hardnessCalcium hardness is one of the five numbers the saturation index needs, together with pH, alkalinity, temperature and dissolved solids.
Step 1: fix the water
- Test pH, alkalinity, calcium hardness and water temperature, and run them through the LSI calculator.
- Bring the index back into Taylor's ideal range of −0.30 to +0.30. Usually that means lowering pH or alkalinity (see pool pH and alkalinity); with very high calcium, lower calcium hardness.
Step 2: remove the deposits
- Use a product made for pool tile and scale, and follow its label for your specific tile and grout (our advice).
- Test any product or scrubbing tool on a small spot first: glazed tile and grout scratch more easily than the scale (our advice).
- Lowering the water a few inches below the line gives you dry access (our note); see the partial drain calculator for gallons per inch.
- Do not mix cleaning products with other pool chemicals (CDC: pool chemicals can injure people when mixed).
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For the deposits Pool tile and scale removerProducts made for pool tile. Check the label for your surface (tile, plaster, vinyl) and test on a small spot first.
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For the deposits Pumice stone for pool tileCheck that the product says it is safe for your tile: a stone that is harder than the glaze scratches it.
FAQ
Why does it come back every summer?
Warm water raises the saturation index. Water that is balanced in spring can be scale-forming in July (see does water temperature change pool balance?).
Sources
- Taylor Technologies: LSI calculator and target ranges (LSI +0.50 and higher: scale on surfaces, ideal range)
- TroubleFreePool: ABCs of pool water chemistry (above 650 ppm calcium causes scale)
- Utah Admin. Code R392-302-25 (public pools) (positive index means scale-forming tendencies)
- CDC: Pool chemical safety (pool chemicals can injure people when mixed; written for pool operators)
Sources read on November 6, 2026, unless a date is given. Product labels change: the label on the product you buy always wins.
