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I have two clownfish that have been in my tank for almost 2 months. They have been fine all this time, eating well, swimming, and hosting at night with my heater. Tonight, I noticed a strange mark on the white band located just behind the eyes on one clown. The mark runs not quite the length of the white band (meaning from eye to eye). It almost looks like some of the white is worn away. It looks, for lack of a better description, like the white is transparent.... like you could see the orange body color through it. I hope my description is clear. Any ideas what this might be?

Thanks.