Gilbert draws its municipal water from a blend of Central Arizona Project (Colorado River) water, Salt River Project surface water, and local groundwater. Gilbert's own annual water-quality reports consistently note total hardness in the 15 to 20 grain per gallon range - meaningfully harder than neighboring Chandler and among the hardest supplied to any East Valley city. Every shower deposits calcium and magnesium onto tile, glass, and grout. The tile face wipes clean; the grout pores absorb it permanently, and the glass etches over time.
The second factor is builder grout. Nearly every Gilbert master-planned community built between 2000 and 2018 - Power Ranch, Seville, Val Vista Lakes, Layton Lakes, Adora Trails, Agritopia, Morrison Ranch, Lyons Gate, Trilogy at Power Ranch - was installed with unsealed sanded or non-sanded grout in the showers. Sanded grout is a hard sponge; in an enclosed, high-humidity shower it absorbs body oil, shampoo film, and airborne mildew spores. That is what "black grout" actually is - biological growth living below the surface of a porous joint.
The third factor is caulk. Every soft joint in a Gilbert shower - pan-to-wall, wall corners, curb, niche edges, and every fixture penetration - is caulked at build with builder-grade silicone that has a 3 to 7 year functional life. Long before the tile itself fails, that caulk dries, shrinks, and separates. Water tracks behind the pan, wets the substrate, and drives mold growth that no amount of surface cleaning can reach. This is why "just cleaning" a Gilbert shower does not last: the caulk has to be cut out and replaced.
For a deeper read, see our guides on the health impact of shower mold, removing hard-water stains from Arizona showers, and grout color sealing, plus the full chart of grout colors we match to.