Pool Era field notes · Care and ownership
Hard Water and White Scale on Pool Tile
Seeing a white, crusty line on your waterline tile? That is usually calcium scale, and it is common here. Our water is hard, and summer heat makes it worse.
Here is why it happens and how to keep it in check.
Why it forms
Hard water plus evaporation.
The San Antonio Water System says calcium carbonate in Edwards and Trinity aquifer water is natural, and its typical hardness is 15 to 20 grains per gallon.[1]
When pool water evaporates, the minerals stay behind. Every top-off adds more. Over time the water holds more calcium than it can keep dissolved. When pH or alkalinity runs high, or the water is warm, calcium comes out of the water and sticks to tile and plaster.
Slow it down
Balance beats scrubbing.
- Keep pH and alkalinity in range. High pH is the most common trigger. Test weekly in summer.
- Test calcium hardness a few times a year. Very low calcium is not the goal either. For new plaster, the National Plasterers Council calls for raising calcium hardness to at least 200 ppm during start-up.[2]
- Wipe the waterline with a soft brush or sponge every week, before deposits harden.
- Cut evaporation with a cover, so you top off less.
- Ask about a sequestering agent. The NPC start-up card notes many recommend one for new finishes.[2]
Removing scale
Go gently, and protect the stone.
Light scale often comes off with a soft brush and a tile cleaner made for pools. Thick scale may need a professional. Strong acids and hard scrapers can etch tile, scratch glass and wear grout.
Natural stone needs extra care. The Natural Stone Institute says limestone and travertine have a calcium carbonate base, can be changed by mild acids, and vary widely in how much water they absorb.[3] Keep acid cleaners off limestone coping, and ask whether your stone should be sealed.
When to call
Some scale means a bigger fix.
- Scale is thick, gray and rough even after cleaning.
- The plaster feels like sandpaper.
- You keep fighting cloudy water and high pH.
- Your coping is flaking or pitting.
Pool Era can test the water, look at the tile and stone, and tell you what it needs.
Related
More field notes
Finishes and renovationPlaster, Quartz or Pebble: Choosing a Pool Finish in the Hill CountryHow plaster, quartz and pebble differ in feel, lifespan and upkeep with hard water.Read 
Finishes and renovationThe First 28 Days After New Pool PlasterFilling, brushing and chemistry for a new finish, step by step.Read 
Care and ownershipPool Water During Hill Country Drought RulesHow drought stages work, and what they mean for filling and topping off.Read
Sources
Where these facts come from
- San Antonio Water System: Water quality FAQs (hardness)
- National Plasterers Council: Pool Start-Up Procedures card (2023)
- Natural Stone Institute: Limestone
Sources checked October 25, 2026. Rules, fees and water stages change; check the source before you rely on a number.
Before you rely on this. This field note is general information, not professional, engineering or legal advice. Every pool and every site is different. What Pool Era does on your project, how and with what warranty, is only what your written agreement says. Local codes and HOA rules change, so check with your city, county or HOA before you rely on anything here. Website terms
