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Bigger fish. I have a red tail shark with 2 rainbows and 2 silver dollars. It does bug the smallest rainbow when I feed...but only for about 5 minutes while the feeding lasts...other than that it doesn't bother the fish.
Are you getting multiple red tail sharks? those would have to be housed in a long aquarium where they can make their own territories. In some cases they become very aggressive towards each other.