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i cant see the bone. its wierd it is always darker than his apperant color. my tank is only 60gal :-( i am getting a 180 for him but it may be a couple of months. it looks like a blister that could pop? i will try to load the pic
It is most likely a gas bubble or tissue build-up that forms form them constantly running into the tank. It is not life threatening but can lead to bigger problems. If it keeps running into the side of the tank it will most likely crack its cuttlebone and possible die.