The Manny is no more aggressive then any other piranha.. Anyone buying a piranha for aggression should switch to a different fish. You run the same chance of having the shy chill in one spot piranha with any species. I think what makes the Manny so desirable is because for most people it's the best looking fish. I loved mine and when the right one comes along I will have another. For me it's looks alone that makes the Manny my holy grail. I don't agree they are harder to keep alive, they require the same care as any other piranha. If you take care of your tank and your filtration you will have no problem keeping them, they just don't handle pH changes or Ammonia as well as most fish since they come from very oxygen rich, fast moving water. They are amazing fish for sure and I won't make the mistake of letting another one go. I did it once and it won't happen again lol. Trigga is right.. they don't make the trip from shipping after they get over 6" and usually die. Even at 6" if you have one shipped to you, you will most likely spend your first few weeks healing your fishes fins, eyes, and body back to normal health. They don't handle it well at all. The only option to get a big one is to grow it out and they grow slow. Most inexperienced fish keepers don't offer it a good diet, good water and take the time to grow them out.. so most don't see past 8"
