Completely clear 1/3 of the tank lenght without any live rock, etc..... Lure the bad fish over the clear side, put a divider in. Catch the bad guy. Learned this trick from a friend, work everytime and saved me lots of time with large tank and lots of LR.
You can purchase a tank divider from LFS such as petsmart, etc... or cut a piece of acrylic to make a divider, then glue a rectangular base at the bottom, so when placing this divider in tank, it will stand by itself.