Way back in the day, the wife and I were at the LFS and she saw a Fuscia (purple) pseudochromis and just had to have it despite my misgivings. Got it home and turned it loose in my 60 cube. As I suspected it would be, it became a bloodthirsty killer in short order. It took me a couple weeks to catch it, and when I did, it went in to my 150 that, at the time, was home to a 4' Undulated Moray. "Killer" made it for several months before disappearing. I did not mourn his loss.
Like Nina said - impulse buys RARELY work out well.