There are hundreds of different fish you could put in.

It depends on the type of fish are offered near you and what type of tank you are looking for.

If you are looking for a tank housing more aggressive fish, I love the look of tiger, ruby and odessa barbs. They are know to nip fins of other fish, but they should be fine together.

If you are looking for great reproducing fish, livebearers are an amazing choice. Guppies are very colorful, and platys, swordtails(my personal favorite) and mollies are good choices. Mollies and "swords" get a little larger, but if you do like mollies, balloon mollies are almost a dwarf of the regular mollies with most of the same colors.

Small community fish of choice are tetras and danios. Danios are EXTREMELY cheap, and EXTREMELY hardy. With a 15g tank, I recommend small tetras. You can pick between neon/cardinal-which are not very hardy, silver-tip, glo-light, black neon, and gold tetras. All of these are very good choices.

If you are looking for a little bit larger fish, Australian Rainbowfish(my profile pic) and Skirt black Widow tetras,and gold skirt are my favorites. Just remember, the rainbowfish only turn really colorful during spawning, but they are cheap and hardy.

Good Luck. Hope this helps. Let us know what you get!