Your tank it too small for the fish you have in the tank. (I suggested to you in another thread that you should go ahead and move them into their bigger thank that you have planned for them)
this fish is stressed due to the lack of available territory in the small tank, it is trying to fight for his/her section of the tank. It is only going to get worse before it gets better in that tank. The dominant fish will slowly start to kill off the other fish in the tank.