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Kole tangs need a surprising amount of vegetable matter to stay fat. I keep a almost constant supply of romain lettuce and nori sheets clipped into the tank. The Kole is a constant grazer, as most surgeonfish are.
Another reason they need a tank that is large enough and has an aggressive filtration system to handle the large amount of poop surgeonfish create. Vegetable matter diets and short digestive systems can make a mess quickly.