Fish aggression

[Reefugee]

Hello,

I have a 120 gallon with a mate pair of percula clown (1.5" and 3"), a yellow tang (3"), a hippo blue tang (3"), a lawn mower blenny, and two chromis.

This past week, th female clown has a gash on it side. Then today, it has a pretty deep cut under its chin. I cannot figure out which fish did it. The clown acts skittish around the hippo blue tang, but I think it's the yellow tang that is causing the problem. Unfortunately, all the damage occurred when I am not observing, so I don't know what fish is causing the problem. My GF things it's the hippo blue that is causing the injury - but I think it's more likely to be the yellow tang. I thought the hippo blue were pretty docile, while the yellow are aggressive. Comments?

Thanks
Minh