OK, I am an advocate for only one fish that will thrive in a tank this small and do very well for people
The fish I recommend is the Heterandria formosa, aka the least killie or dwarf live bearer. This guys are hard to come by, these are usually a special order from your pet shop but they are great fish.
The males reach about 1/4 of an inch long at full adult hood and the females are larger at an inch. They are very peaceful fish and thrive in tiny tanks. I own a small colony right now in my 5 gallon. I plan on getting more soon for an outdoor container. But they do well having plants like hornwort floating on the top of the tank and they eat crushed fish flakes. Many breeders keep them in 1 gallon pickle jars and they thrive in them. Since they are native to the southern US, they do not require a heater for their tank. I would say 15 would be perfect for this size tank.