Get a Serrasalmus Rhombeus if your only looking one piranha. If you want a shoal of them get six Caribe. A 100 gallon for a Serrasalmus Brandtii is more than needed and you would just be wasting room. But it's up to you. I imagine if you did put a Serrasalmus Brandtii in there it would grow nice and big because it would have more room to swim in. Probably could grow over it's max size because they only need about 40 gallons per Brandtii but they can't be kept together. They would kill each other.
if it is a 100ltr it will be @ 40 us gal so getting a serra will probably be best and as he asked for only 1 piranha then a brandti would be a good choice or a smallish rhom say 5-6'' .
if it is a 100 gal then i would go with pygo's either reds or cariba but if moneys not tight then piraya @ 6-7