Clown now fighting after approx 3 yrs

[TRIX]

Hey all.

I have a RSM.
Livestocke
2 clown fish. (nemo's)
1 starfish
1 long spine urchin
1 sea cucumber

One of the clowns has always been territorial and hosting the mushrooms but never letting the other one in. Now the other clown has decided to host the clam shell that I have. The original boss clown will not leave the clown that is hosting the clam shell alone. It gets really bad at feeding time. They fight so mush they don't even get the food.

I've thought about adding another fish to change the dinamics of the tank.
I will be moving in about 3 months so I don't want to do anything major but want to solve the issue.

Any suggestions?