Though many stains fall into similar categories, some are so odd that they almost seem to have come from another dimension. Here are a few quick stories of bizarre stains, and how experts diagnosed them.

The expert: Danny Cervantez, business development representative at PoolCorp in Van Nuys, Calif.

The stain: Bluish streaks resembling stretch marks on skin

The diagnosis: Tests revealed cyanuric acid (CYA) levels above 200, as well as low carbonate alkalinity.

The solution: A partial drain-and-refill lowered CYA levels; a copper stain kit and a sequestrant removed the blue marks.

The expert: Danny Cervantez

The stain: “U”-shaped marks that turned nearly invisible in direct sunlight

The diagnosis: Tests revealed low carbonate alkalinity and low calcium in the fill water.

The solution: Adjusted water chemistry fixed the alkalinity and calcium issues; a metal stain removal kit and sequestrant got rid of the marks.

The expert: Ron Gee, warranty manager at CDC Pools Inc. in Chandler, Ariz.

The stain: blackish or purplish dots too isolated to be algae

The diagnosis: Tests revealed iron coming in with the fill water.

The solution: Hydrogen peroxide cleared up the dots; adding a sequestrant protected the pool against their return.