I have 20 brilliant rasboras in a 120 gallon planted aquarium for about 8 months. About two months ago I saw a small dark growth on the head of one. It has been growing. He seems to be able to eat and acts naturally, but I am concerned. Can anyone help identify what it is? I am hoping it's just a tumor and not contagious. Non of the others have it as far as I can see. Thank you
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