Need advice on injured fish

I lost my McCosker's Flasher Wrasse a week and a half ago to a freak injury. I found him dead with a big slice in his side as if he were cut with a filet knife. Now my Red Lined Wrasse has a less severe but very similar slice. I think it is superficial this time and the fish is acting fine, but I'm not sure what is happening.

I have an MP10 powerhead, but I don't know if it is big enough to cause this type of injury. I have had the MP10 for 4 months and this never happened until 10 days ago. Has anyone else had this happen? Is there anything I can do to make it less likely? Here are a couple pictures that aren't really the greatest. He is in constant motion and hard to photograph.

 

Do you have an aggressive tang?

 

My Kole Tang used to be a bit of a jerk but he has been pretty well behaved for the last 6 months or so. I had a black combtooth blenny that he hated and a lawmmower blenny that he hated, but he has never bothered my other fish.

 

My two tangs will show up with slices from time to time from fighting each other.

 

Thanks! I'll keep an eye on him. I wouldn't put it past him.

 

Not anymore...

My advice: remove your aggressive tang.

 

Thanks! If you are confident he is the culprit, I will remove him. Is it wrong for me to sell him back to my LFS if I know he is aggressive, or do you think a change in scenery will make him less aggressive. I'm not sure about fish returning etiquette.

 

nothing wrong with selling him back as long as they will buy him.