you people do realize that there are thousands of tetra species that have all sorts of temperaments, eating habits, and maximum sizes, right? saying "its silver with red and black fins" means essentially nothing and could in theory describe any number of fish, including many which aren't even close to being tetras.
i wouldn't be surprised if you're right about it being an X-ray Tetra, but its still not a good practice to identify fish without more thorough descriptions. other fish that are "silver with red tails and black/yellow fins" could be White Cloud Mountain Minnows (coldwater fish), Semaprochilodus taeniurus (grows to be a foot long), or even Chalceus erythrurus (also a foot long but with teeth)!