Nine out of ten times these fish are imports being caught in a natural environment, held and shipped to the states (or other country), then held by the dealer before they're purchased and taken home or quite possibly purchased and shipped again to a hobbyist from a far away location. What happens frequently with new piranha hobbyists is a kind of profiling that stems from many different origins...possibly from other inexperienced piranha keepers, salesmen or even Youtube videos that highlight how aggressive these fish are.  This profiling leads to a false understanding of what they are getting, which in turn leads to impatience, then frustration and then finally a lot of times giving up on the young Serrasalmus which can and many times does, go down bad roads...

I am in no way an expert on these awesome predators.  What I am is a hobbyist that got started with these fish many years ago, and learned through experience what to expect, what is fact and what is fiction.  My only reason for this write-up is to hopeful clarify these animal's true behavior so that the new hobbyist has a better understanding of what he/she is getting into, hopefully BEFORE they purchase the animal, and is able to decide if this fish is right for them.

Are these fish aggressive?  YES
Are these fish territorial?  YES  
Do they show this behavior immediately?  Not likely.

What one has to understand is that, especially when juveniles, Serrasalmus can be EXTREMELY skittish, sometimes to a point where it begins to feel to the hobbyist as if he/she is keeping an empty aquarium.  This skittishness can last a very long time.  Personally, the longest I've ever dealt with an extremely skittish Serrasalmus was over a year, however, I have read accounts of this skittishness lasting more than two years, so its easy to see that if the new hobbyist is not expecting this, it can and will become frustrating.  This behavior is absolutely normal and I cannot stress enough that it should be EXPECTED of the juvenile Serrasalmus to act in this way.  The reason for this behavior, summed up, is survival.  When these fish are young, they're what are called ambush predators, meaning they spend their day hiding amongst aquatic foliage, driftwood, submerged root systems, rocks and other hiding places, waiting for a quick and easy meal to come within grabbing range.  At the same time, they're constantly trying to avoid becoming a meal themselves to larger predators.  This is what they are wired to do in the wild, so when they're introduced to an aquarium environment, their natural instincts are what they go by and, coming from the fish's perspective, the hobbyist leaning in close to the front panel of the tank looks like a huge predator!

Not every individual Serrasalmus displays this behavior.  There are some that become outgoing quickly, but in my experience, the vast majority need time to come around.  Some take longer then others...some take weeks and others take years, but the best thing the hobbyist can do is provide a good quality of life and be patient.

Many experienced and novice hobbyists alike have tried many different routes to fast track their Serrasalmus to show more personality.  These include: 
   - Keeping the fish in a bare tank, thus forcing him into the open.  The idea is that over a short period of time, the Serrasalmus will realize that the hobbyist poses no danger to him, and he will quickly come out of his shell. 
   - Packing the tank with many hiding places, and allowing the fish to hide as much as needed...allowing it to grow out of this ambush predator mentality naturally over whatever period of time it takes.  I personally have always used this tactic and I've found that although it may take the Serrasalmus longer to come around, depending on the individual fish, this way is far less of a shock to the animal.
   - Some people introduce dither fish (*typically small, fast, schooling fish that act as targets for the primary fish to interact with. Causing them to come out into the open more frequently).  Personally, I feel that this is counterproductive if you're looking to get your Serrasalmus out of hiding, due to the fact that it triggers the hunting instinct, making the Serrasalmus try to become invisible as he waits for the perfect ambush opportunity. 
The new hobbyist will most likely see many keepers of adult Serrasalmus rhombeus  being housed with small schooling fish without issue.  Keepers of juvenile Serrasalmus, even those specimens that spend day after day hiding, should understand that their fish are extremely fast and extremely aggressive to other fish, particularly those small enough to create a prey drive.  It goes without saying though, that equally bad things can happen to fish that matching the Serrasalmus size.  Many people have had horrible experiences when attempting to house two Serrasalmus in one tank with a tank divider in place, keeping them separate;  Tragedy occurs when the divider is knocked down and their territorial drive kicks in.

In short, I do not recommend tankmates for juvenile Serrasalmus and if one chooses to go forth and try, casualties should be expected.

Some of the practices that the new keeper can rely on to aid him are are also some of the most basic:
   1) - Keep your juvenile Serrasalmus tank in a high traffic area of your home.  Get him accustomed to seeing people all day everyday.  My favorite spot that I've placed a tank is where it's at right now...directly behind and butted right up against my couch.  The design of my living room, thankfully, permits me to do so, and I've been very pleased with the outcome.
  2) - Place tank lighting on a timer.  The rhythm of lights on, lights off at the same times daily best simulates a day/night cycle.  (With very newly introduced Serra into the aquarium, my personal practice is to leave the lights completely off for the first 3 days or so that I have him home while he acclimates to his surroundings.)
   3) - Purchase a good quality heater that keeps the the tank water at a steady temperature.  Avoid temperature swings.
   4) - Keep clean, healthy water through good filtration and weekly/bi-weekly water changes.

  5) - A healthy varied diet of prepared foods.  There have been plenty of great, informative articles and threads written on this subject so I'll not be diving in  much here, but what I will say is that it is common for these fish to only accept live foods in the beginning, often being quite stubborn about what the keeper is offering.  The reason for this is that while these fish are being held by the dealer, most feed live feeder fish of some sort for the ease of getting the piranha to accept the food offered.  It is not uncommon for the Serrasalmus not to accept anything other then live once it's brought home.  There is no need for the keeper to worry if the Serrasalmus just will not take the chunks of frozen tilapia....offer the food and remove it, if uneaten, after an hour or so to prevent contaminating the tank water.  The Serrasalmus will not starve himself....do this every few days and eventually (might take a week or two) it will take the food.  It is quite impressive how long even a 4 inch Serrasalmus can go without eating even a morsel of food so a few weeks will not hurt it.  (Similar to the lighting topic, with very newly introduced piranha, I withhold from feeding for a week or so while the fish acclimates.)

All of these steps can be taken to help ease the newly imported Serrasalmus into life in an aquarium, but the fact of the matter is that nothing is going to change the behavior of an aquarium fish overnight.  As previously stated, quality of life, time and patience are the keys to successfully raising these fish from juveniles.  With many, many individual Serrasalmus, there are no shortcuts.  As I've stated before in many other threads, I have never kept a single Serrasalmus that was not curious about what was going on in his tank and in the environment surrounding his tank.  Did they start out that way?  Absolutely not!  Some were more difficult than others, but with all of them, I initially dealt with long periods of (sometimes extreme) shyness until a bolder personality developed.

 

Another topic that I've often seen is an aquarist asking how he can train his Black (Serrasalmus rhombeus) Piranha to be more aggressive, like videos seen online.  This is actually quite a comical question to me--persons asking this question should come to terms with the reality that their Serrasalmus is a fish, not a German Shepard.  These fish are aggressive, especially as they mature in an environment their comfortable in, but if having an extremely aggressive predatory fish is all the aquarist is looking for he/she is better suited looking outside of keeping Serrasalmus piranha.  I and many others understand the attraction to having an aggressive fish that chases everything relentlessly, however the amount of time needed to develop that boldness in a young Serrasalmus is often much longer then a casual hobbyist is willing to commit.  Then there's always the possibility that the extreme aggressiveness might never develop as well.