My big sailfin blenny in the 55 is quite boisterous and defends his diatoms and algae well.
I had a tang in there that ended up with parts of it's tail missing (with cookie monster shaped bites out of it) and after weeks of wondering, one day we saw the blenny fly out from behind the rocks and take another bite of Tang's tail. You see, the tang was eating "Blennie's" algae.
SO the tang has long since been moved to a different tank. But last night I was moving some rocks around and decided to poke around at a little algae near the back of the tank and guess who attacked my hand!
Kinda surprised me when he rammed into the palm of my hand and tried to take a big chomp. LOL
I jerked my hand out of there and went AUUGH! and wife was nearby going "What happened!?" etc. It didn't hurt but it sure did surprise me. He is fat but fast.