Answer: Minerals in the water accumulate and leave a "ring" where it evaporates; in most cases it does no harm and would only be of concern because it is unsightly. Keeping the pond topped up along with some overhanging plants can be used to hide it. If it really bothers you, you might try rubbing gently with a pumice type stone made for removing the ring around the toilet bowl being VERY careful not to damage the pond liner. I would be very careful of using any chemical remover near plants or fish -- most of them are fairly caustic and might damage the liner. If it is a decorative fountain you could used softened or distilled water in it but this would not work for a pond stocked with plants or fish because of the salts used to treat softened water. (Distilled water would be awfully expensive for a pond, too!) If it is a common problem in your area you might check with a local water gardening specialty supplier and see if they have any more suggestions.
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