A 2.5 gallon tank is (as the others have said) is really too small for most fish, as they like to be with friends....
Male Betta Splenden (Siamese Fighting Fish) would be great (only one male per tank though) and are happy with the smaller tank and on their own.
Or
What about planting the tank quite heavily and having a couple of really small fish like Tetras - you could have 4 of these,
Or
Bumble Bee Gobies - most of these like Brackish Water, but you can get freshwater ones.
Or
Pencil Fish - these are quite long (1.5") but produce so little waste you could probably have 5 or 6 in your tank.
Let us know what you decide on.
Good Luck
Neil.