Rust color smear on Yellow Tang?

[jgargano]

My Yellow Tang recently aquired a rusty(blood color)smear mark on his dorsal fin.. is it possible he got injured? mabe its a blood blister?

 

[jgargano]

Question for you Terry, why is a water change so important? What do you think It could be? I have been giving my fish their daily allowance of Zoe.