I would try to remove the yellow for awhile so the purple can settle in. Put the yellow back in and put a small mirror up on the side of the tank the yellow stays in so he will attack him self instead of the purple.
LOL...I like the mirror idea...that's funny...who would have ever thought of that!?!
BUT...the Yellow is not coming out! He's been tagged as "Uncatchable without removing everything first", and I don't want to go there.