I would bring it back or get another tank (a large one would be preferable). Plus if it stays where it is, its going to eat your guppies in about two months!
This leads back to the case of the impulse buy!! It is very important to know what you are purchasing before you part with your cash. I'm being hypocritical here as I have bought fish in the past that I hadn't intended to on the day but it was only in the knowledge that the fish species would suite my setup (books in the LFS, personal experience). You should never buy a fish going on what a worker at a LFS tells you, unless you trust him (even them I wouldn't). If you liked the look of a fish the chances are that the LFS will always have more of them in stock so you should take the latin name and ask people on this forum before you buy. It will save you the hassle of going back with the unsuitable fish and reduce the stress on the fish.
Regards