Definately not a rainbow or redtailed shark, wrong body and mouth shape to be a Labeo species.
From what i can see in the tiny picture it looks to be a Orange finned loach; Botia rubripinnus, though there a few similar looking species so that is not an exact ID but is the most likely as this is the most commonly traded species.
They are a fairly large growing loach species which reach around 8 inches quite easily, as adults they are pretty aggressive and will attack other loaches and similar looking fish so unless you have a very large tank then more loaches is not recomended. They will also eat smaller fish such as guppies and tetras and have been known to eat the eyes of other bottom dwellers like Corydoras, so all in all not a great community fish.
