What about removing the undy to a seperate tank for a couple weeks maybe while youadd another aggressive fish in so he establishes a territory first? Or try the old eggcrate trick, but honestly, I don't think it will work. These are just suggestions, really I believe the undy should be in a tank all by itself unfortunately, or I would have one. I wonder how they do in the ocean, there must not be a fish within a few square yards of them!