You've got the plague... Some say they're not a problem, and eat algae and whatnot. Others claim they strip coraline algae, leaving ugly whitish blothes where there was purple/red/pink before.
I'm one of their detractors, and pull a bunch of them out every so often. I don't like the blotchy look they leave behind.
Not sure if their (supposed) benefit has ever been documented either...
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Alan
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