It’s not unusual to see a variety of fish performing the same maneuver as this chub. I think what it’s doing is rubbing off parasites that attach themselves to fish. They seem to like this particular rock, possibly because of its rounded top and just the right amount of abrasiveness.
Graham's Island 
These are so amazing. Let’s follow each other. Thanks Anita
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That’s very interesting!
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It’s kind of fascinating to watch.
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You would think that everyone using the same rock would be parasite swapping rather than cleaning
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I guess that’s a possibility, but they make quick passes and I think the parasites get washed away.
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