Alge stains on a plaster pool

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First, make sure it really is an algae stain. Rub a trichlor tablet on the stains. If they fade, then it is algae. If they do not fade, it may be a metal stain. In that case, rub or hold a Vitamin C tablet against the stain for 30 seconds. If it lifts, than it is a metal stain. Perform these tests and report back on the results.
 
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