Avoiding discoloration in swimming pool

:Q. Our vinyl-lined swimming pool has discolorations on the bottom, but it is not algae. We took a water sample to a lab and were told the stain was due to iron in the water.

Different chemical processes have not noticeably lightened the stain. If we get a new vinyl liner next year, can you suggest anything that will avoid this discoloration problem?Is there another kind of liner that holds up better in this climate? Mrs. Delcie D. Bean Jr. Jaffrey, N.H.

A. If not algae, then indeed the most likely reason for the stain on the vinyl liner is related to metal in the water. Periodically test the water for metal content, thus keeping track of, and verifying, the metal content.

Commercial fertilizer, if allowed to get into the water, could contribute to the staining, too. Avoid getting grass or shrub fertilizer into the water.

Retail swimming pool stores have vinyl cleaning chemicals that might be useful. Follow the label directions as well as professional advice. The usual dosage is one quart of cleaner in 10,000 gallons of water for a metal level of up to 1 part per million (p.p.m.). Use slightly larger amounts of clearner if meltals exceed 1 p.p.m.

Add the chemical directly to the pool water in areas of localized metal stain. Temporary clouding of the water is likely for a couple of hours. Dont's worry about it. For maximum effectiveness of the product, apply to the pool water in the absence of direct sunlight and when chlorine levels are 2 p.p.m. or below.

Where large metal stains have aged into the pool liner, draining the pool and cleaning the vinyl liner directly may be necessary, but that can be costly because of the price of water.

Also, it may be against public policy if water is in short supply. Consult your local swimming pool company for on-the-site advice.

Pool Magnet, made by Bio Lab Inc., of Decatur, Ga., inhibits stain and scale caused by iron, copper, manganese, and the like. Your pool-supply store may recommned an equivalent.

As to various types of swimming pool liners and which is best for your New England climate, get in touch with a reputable swimming pool company or store. Usually, pool liners are not replaced until they leak, so as to get the maximum mileage out of the investment.