Then you have a problem.
Serpae tetras,
Hyphessobrycon eques, as others have mentioned, are notorious fin-nippers. So is the closely related tetra
Hyphessobrycon callistus. In fact, unless you know better, it's safe to assume that any barb or tetra is a potential fin-nipper until you have confirmed otherwise. Most are not fin-nippers, but enough are that it is worth taking this cautious approach if you only have a single aquarium.
In some cases fin-nipping is a side effect of not having enough fish in the school. tiger barbs are classic examples of this. In other cases, the fish are naturally "parasitic", and feed to some degree on the fins of other, often larger, fish. I believe that many tetras fall into this category. A few years ago, I wrote an article for TFH magazine called "Fish no-one should ever keep", and the serpae tetra was number three on that list!
Besides guppies, other fish to avoid mixing with fin-nippers are angelfish, Siamese fighters, gouramis, fancy goldfish, and basically anything that is slow and has long fins.
Some aquarium stores will take back fish. Ask nicely, and don't expect any credit. Some stores will give credit, but don't expect it, especially not on two serpae tetras.
I'd suggest buying a nice book like Baensch's Aquarium Atlas; it is very useful to have a book to hand for when you go shopping, so you check up on social behaviour before buying the fish.
Cheers,
Neale
I haven't got another tank.