Dissolved metals and minerals are present in every swimming pool. These dissolved solids find their way into a pool water in a number of ways: Fill water from wells or from city water lines can deposit dissolved minerals into the pool. Galvanized pipes and bronze or copper fittings in pool pumps and heaters are sources of iron, copper, and manganese. Foreign objects, from bobbie pins to pine needles, leave behind trace metals and minerals. Evaporation removes water from the pool, but leaves minerals behind, gradually raising the pool water’s total dissolved solids level. Over time, these dissolved solids will precipitate, or “fall out” of the pool water, and settle into the suface of the pool. The resulting rust orange, brown, grey, and light blue-green stains are unsightly, and detract from the beauty of your swimming pool.


  • Periodically draining and acid washing the pool surface.
  • Mechanically removing minerals before they have a chance to deposit on the plaster. The type of filter required to remove minerals and heavy metals is called a nanofilter. It will clean the water to “bottled water purity” without having to drain water from the pool. Once the pool water has been cleaned with the filter, a chelating agent can be used to maintain the water in a “soft” condition.
  • Chemical treatments, which remove minerals before they have a chance to settle on pool surfaces. These chemicals are called chelating agents. Their name comes from the Latin word for “lobster claw.” Chelating agents “pinch” dissolved metals and minerals, holding them until they can be strained out of the pool water by the filtation system. Chelating agents are also known as sequestering agents.


Draining and acid washing the pool will eliminate many stains, but it also accelerates pool plaster disintegration. Done by untrained or incompentent technicians, acid washing can lead to irreparable damage, from etching or pitting of the pool plaster to cracking of the pool shell. The cost and quality of acid washing vary tremendously. Houston area pool companies typically charge between $125 and $600 for an acid wash! Since there is little regulation of the pool repair industry, you can’t always be certain of the competence of the crew performing the acid wash — the less expensive companies may actually provide the highest quality service! Whatever the cost, you must pay the expense of draining and refilling the pool, which typically runs in the $100 range. Acid washing removes stains by dissolving a thin layer of pool plaster. The short-term result is whiter, cleaner plaster, but the long term effect– rougher, more stain-susceptible plaster — makes the process very unappealing.

Nanofiltration costs $80 per day. Most pools require two or three days of nanofiltration. The nanofilter removes 300 to 400 parts per million of dissolved solids per day, greatly reducing the possibililty that metals or minerals will “fall out” of the water. The nanofilter is a portable unit, with its own pump and plumbing. It must be set up on your pool deck, making for an unattractive temporary addition to your backyard. Since dissolved solids are constantly building up in the pool water, nanofiltration must be done every 6 to 8 months.

There are a variety of chealting agents on the market. Pelican Pool Service offers YR-17, a very effective stain prevention treatment. Developed by a team of chemists and pool professionals in southern California, one gallon of YR-17 should be added to the pool every 4 months, all year ’round. While YR-17 will not remove existing stains, it is an outstanding means of preventing new stains from forming.


Whatever stain prevention treatment you choose, there are costs invloved. For example:

One acid wash —
$125 to $600 (with accompanying plaster disintegration) + $100 water refill costs = $225 to $700
1 year of nanofiltration —

About 4 days per year, @ $100 per day = $400
1 year of YR-17 —

3 gallons @ $35 per gallon = $105


Regular maintenance is the best prevention against staining. Regular use of a chelating agent will prevent stains, preserve the appearance of your pool, and save the headaches and risks associated with nanofiltration and acid washing.

Some surface discolorations are not stains at all, but result from flaws in the pool plaster. For information on calcium chloride spotting, crazing, spalling, and other flaws, please see our plaster problems page.