| This could be an example of allelopathy which is the release of chemicals from one coral to protect it's territory and win itself some new "land". Nothing has to be touching. Two corals could be "fighting" which could affect other corals in the tank and not just the ones directly involved.
Don't put chemically aggressive corals upstream from others and use carbon and change it regularily. Make smaller more frequent water changes to dilute the chemicals as much as possible.
My leathers are always picking on each other.
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Renee | 
