Short answer: the white or gray crust on Phoenix pool tile is mostly mineral buildup, mainly calcium. It comes from two related causes. One is evaporation at the waterline, which leaves dissolved minerals behind on the tile. The other is calcium carbonate scale, which forms when pool water is oversaturated with calcium carbonate (usually from high pH, high alkalinity, high calcium and warm water). You can’t stop evaporation in the desert, but regular brushing, steady water balance and managing dissolved solids over time slow the buildup a lot. Heavy, hardened deposits usually need professional removal.
Why Phoenix tile gets crusty
Phoenix tap water is hard. The City of Phoenix’s 2025 Water Quality Report lists total hardness in its water system at 172–302 ppm, or 10–17.6 grains per gallon. The city explains that hardness comes from minerals such as calcium and magnesium that the source water picks up from soils on its way to the treatment plants. The city also notes that hard water can cause scaling on pipes, water heaters and plumbing fixtures. Your pool is no different.
A pool loses water to evaporation constantly, and evaporation removes only water, not minerals. Each top-off with hard tap water adds more minerals. Over months and years, calcium and total dissolved solids (TDS) in the pool tend to rise. That’s why the tile line in Valley pools can build up faster than in places with softer water and less evaporation.
Two different deposits that look the same
It helps to know which one you have, because the fixes overlap but aren’t identical.
1. Waterline (evaporation) deposits
An article in AQUA Magazine explains that pool water wicks up onto the tile face, typically about 1/4 to 3/4 inch above the water level, depending on how porous the surface is. That thin film evaporates and leaves its dissolved solids behind. The author points out that this happens where there is no standing water, so it isn’t strictly a water-chemistry imbalance. It can’t be prevented just by adjusting the saturation index. It can be slowed by lowering the water’s calcium and dissolved-solids levels and by reducing evaporation.
What it looks like: a band of white or gray crust sitting at or just above the waterline, often thicker on the sunny or windy side of the pool.
2. Calcium carbonate scale
Scale forms when the water itself is oversaturated with calcium carbonate, so the mineral comes out of solution and deposits on surfaces. The industry measures this with the Langelier Saturation Index (LSI). According to a technical bulletin from the Pool & Hot Tub Alliance (PHTA), the LSI combines pH, temperature, TDS, carbonate alkalinity and calcium hardness:
- LSI around 0: balanced, neither scale-forming nor aggressive.
- LSI above 0: potentially scale-forming. The higher it goes, the stronger the tendency.
- LSI below 0: potentially aggressive, meaning it can etch plaster and grout and corrode metal.
PHTA’s bulletin gives −0.3 to +0.5 as the minimum-to-maximum LSI range for proper pool water balance (citing APSP).
Warm water raises the LSI. Orenda notes that calcium hardness on its own usually isn’t the main driver of scale unless levels are extremely high. It usually takes a combination of factors, such as high pH, warm water and high carbonate alkalinity, to push calcium carbonate out of solution.
What it looks like: rough, hard deposits below the waterline on tile, steps, return fittings or inside equipment such as salt cells and heaters.
How to slow buildup down
None of these are one-time fixes, but together they make a real difference.
- Brush the tile line regularly. Fresh deposits are soft and come off far more easily than hardened ones. Make this part of every weekly service visit. Here’s what a weekly visit should include.
- Keep pH in range. CDC recommends pH 7.0–7.8 for pools, mainly for germ-killing and comfort. Letting pH drift high also raises the LSI and the tendency to scale.
- Test calcium hardness and alkalinity, not just chlorine. Ask your service to track these and calculate the LSI, especially in summer when water temperature climbs.
- Watch TDS over time. As minerals build up from evaporation and top-offs, partially draining and refilling may eventually be recommended. Whether and when to do that depends on your pool surface, the water readings, the season and any local water rules. Ask a professional rather than guessing.
- Reduce evaporation where practical. Less evaporation means fewer minerals left on the tile and less hard water added at top-off. ENERGY STAR lists water conservation among the benefits of a pool cover, so a cover is one option to discuss with a pro.
- Ask about scale-control products. Sequestering and scale-inhibiting products are widely sold. Use them only as the label directs, and understand they’re meant to help prevent new buildup, not remove heavy existing deposits.
Removing existing scale
Light, recent deposits can often be removed with a tile brush and a cleaner made for pool tile, following the label exactly. For heavier, hardened crust:
- Avoid metal scrapers and harsh abrasives, which can scratch tile or damage grout.
- Be careful with acids. Acid-based tile cleaners and acid washing can damage some surfaces and need careful handling. Never mix pool chemicals together.
- Consider professional tile cleaning. If you hire someone for tile cleaning, ask what method they use, whether it is safe for your tile and grout, and whether the pool needs to be partly drained.
If you see buildup inside a salt cell or heater, have a professional check it.
What’s specific to the Phoenix area
- Hard water in, minerals stay. With city water hardness in the 10–17.6 grains-per-gallon range (per the 2025 Phoenix report), every top-off adds calcium.
- High evaporation. Hot, dry, windy conditions mean more wicking and evaporation at the tile line.
- Warm water much of the year. Higher water temperature raises the LSI, so water that is balanced in winter can lean toward scaling in summer.
- Your city may differ. Hardness depends on your water provider and source. If you are not on City of Phoenix water, check your own provider’s annual water quality report for the current hardness range.
When to call a pro
Call a licensed pool contractor if deposits are thick or hardened, if you see scale inside equipment, if you’re considering a drain-and-refill, or if the tile or grout looks damaged. Tile cleaning and water balancing fall under service work, while re-tiling or resurfacing is remodel work. Ask what license class the contractor holds and check it with the AZ ROC. Browse pool service and repair contractors or pool construction and remodel contractors, or start with your city: Phoenix, Scottsdale, Tempe, Glendale or Peoria.