With a 125, i would go for a mix shoal of caribe, terns, pirayas!!. All piranhas act different individually, agressiveness depends on how hungry, adaptation to surroundings, and the piranha itself. But if you would like only one piranha, go for a diamond black rhom!What P would be good for a 125g. Single or a shoal. Also i want it to be aggresive an not jus hide like some piranha i have seen.
But don't forget filtration, fish size, fish species, and size of tank. I could do 10 water changes a day and theres no way a 19 inch piraya could live in a 1 gallon tank."Frequency" and "Volume of" water changes are the largest dictators of how many fish can be kept in a tank.