Here is the difference:
Piranhas are normally about 15 to 25 cm long (6 to 10 inches), although reportedly individuals have been found up to 43 cm (18.0 inches) in lengthSerrasalmus, Pristobrycon, Pygocentrus, and Pygopristis are most easily recognized by their unique dentition. All piranhas have a single row of sharp teeth in both jaws; the teeth are tightly packed and interlocking (via small cusps) and used for rapid puncture and shearing. Individual teeth are typically broadly triangular, pointed, and blade-like (flat in profile). There is minor variation in the number of cusps; in most species the teeth are tricuspid with a larger middle cusp that makes the individual teeth appear markedly triangular. The exception is Pygopristis, which has pentacuspid teeth and a middle cusp that is usually only slightly larger than the other cusps. In the scale-eating Catoprion, the shape of their teeth is markedly different and the premaxillary teeth are in two rows, as in most other serrasalmines.
This is a piranha caught in Lake St. Clair August 20th, 2007
Here is the African Tiger Fish
The African Tigerfish - Hydrocynus goliath is the worlds largest member of the Characins which consist of all of the fish in the following families tetras, dollars, pencilfish, piranhas. The African Tigerfish is big, powerful, and well armed with big evil looking teeth. These teeth have made the African Tigerfish one of the most sought after exotic sportfish. They lives in the rivers and streams of central and northern Africa.
The African Tigerfish gets its name from the horizontal black bands run along the entire body as much as from the fierce looking teeth it possesses that show even when the mouth is shut.
To add even more to the reputation of this fish besides the size and ferocious look is the fact they despite the large size they hunt in packs just as piranhas do. Luckily they feed primarily on other fish although there are unverified reports of attacks on people.