This is a fish group all people are familiar with as it contains such "vicious fishes" as the Piranhas... as well as the almost strictly vegetarian Pacus and Silver Dollars. Many species appear in pet fish markets, the toothy ones on a State by State basis in the U.S. as there are governmental fears/restrictions about release to the wild. 

    Given water of moderate softness and pH, and starting with initially healthy specimens, the Tetras of this subfamily are quite hardy. With some specialization of diet for most of the piranhas toward the meaty end, all accept aquarium foods readily. 

Range:

These fishes are all South American. Individual species have different ranges as mentioned below under their accounts.

Size:

    The Serrasalminae comprise thirteen genera, of about sixty species, the smaller ones about palm-sized overall, the largest Pacus topping out at more than three feet and thirty kilograms! Including Catoprion, Colossoma, Metynnis, Myleus, Pygocentrus, Serrasalmus. 

Survey of the Groups, Species Most Familiar to Aquarists:

Silver Dollars...

Genus Metynnis: Ten valid species out of 23 nominal. 

Genus Myleus: the "Hook" Silver Dollars; 17 nominal, 13 valid species. The two most often seen shown below: Warning: though these are greens eaters par excellence, they can/do deliver a considerable bite... watch your hands when netting them.

Genus Mylossoma: Ten nominal species, half are valid. 

Pacus, the "fruit-eating" Piranhas:

Genus Colossoma: Five valid species out of eleven nominal. Important food fishes and civil servants delights in the U.S. as more P.R. goes there way with each "piranha" caught in a local freshwater basin... Maybe they're releasing them, hmmm. Also see genus Piaractus.

Genus Piaractus: Two valid species... placed in other genera at other times... (Colossoma, Myleus...). Big plant eaters that can still give you a big bite. 

Piranha Per Se: Catoprion, Pygocentrus, Serrasalmus... About thirty five species. 

Verticals (Full/Cover Page Sizes Available)

Stocking/Selection:

    I use/suggest three principal criteria when selecting these fishes: Time on hand, markings/damage, and the tried and true "feeding test".

1) How long has your source had the fish? Don't buy "new arrivals", even from a retailer. You might be very surprised how long the livestock you're looking at has been in the "chain of supply" (almost all Serrasalmines are wild-caught). It might be just a few days, perhaps a couple of weeks. Best to let these fishes "settle-in", harden to their new captive conditions before moving them on.

2) Look closely at the prospective buys body, fins and eyes. The latter should be "bright" w/o inclusions or cloudiness. The fins, particularly the unpaired ones (dorsal, anal, caudal/tail... the adipose is not really a fin) may have splits in them. This is generally not a problem, and these heal quickly of their own accord, but do look at alls bases for red markings, indications of probable infection. I would leave these in place for curing. The bodies should show no open sores, but you may well come across ones with curious "bumps" of small size. These are indications of Sporozoan disease, that are of little consequence. They don't seem too deleterious, are not spread to others due under aquarium conditions. 

3) A feeding of livestock in front of you tells much. That the organisms are healthy enough to be accepting food. The types of food they're taking, and that there's a good chance they're "going to make" the transition from where they are now to your destination.

Environment

Habitat:

    In order to appreciate these animal's capacity for growth, motion and behavior they need space. As a rule of thumb a good six times a given species average maximum length long and twice in width (information on sizes can be found on fishbase.org). 

    Live plants are a definite plus, but must be either separated by a partition with all but the more carnivorous piranhas (I once saw a large silver dollar eating terrestrial plants outside its tank at the Wilhelma Aquarium in Stuttgart Germany...), shown as contiguous tanks back to back, or linked to the main/display tank with just the water going back and forth.

Chemical/Physical:

    As alluded to above, these fishes hail from waters that are typically softer and of less pH than most municipal waters. If your wholesaler/importers, and retailers have done their jobs, by the time you receive them these "saw-tooth fishes" should be accustomed to dechloraminated tapwater conditions. If not (you can ask, test the water they're in), it's best to "meet them midway with chemicals available in the trade for modifying water chemistry.

Filtration: 

    Should be vigorous on a few counts. Most of these fishes are fast moving, high metabolism types, going through oxygen and food more than small tropicals that you may be more familiar with. Likewise these are messier animals that need circulation, particulate filtration to keep their tanks tidy. Lastly, as the environs most of these fishes hail from are large and free-flowing, they have little tolerance for metabolite build-up (compounds like nitrates slow down their growth, diminish color, vigor overall). Frequent partial water changes, live plant materials (eaten or not), and periodic use of chemical filtrants (like carbon, replaced once a month) are good ideas here.

Behavior:

Introduction:

    Like most other Characiform fishes, these ones are better placed in conditioned water, ideally in systems that are at least weeks old if not months. They can be easily placed with a standard "floating bag" or "drip" protocol in darkened (fish lights off) conditions.

Reproduction:

    Group to pair spawners in season, triggered by rain, lower temperature (known in some species, assumed in the rest). No parental care for sticky eggs of small size laid in plant material. Some species of most genera have been spawned, reared in captivity, even the Pacus, for aquaculture. 

Foods/Feeding/Nutrition:

    Though there are distinctions meat versus vegetarian, amongst these fishes, the actuality is that they both eat flesh and greens... and should be offered a bit of both, more meat for the "real" piranhas (some as you'll find are more vegetarian), and less for the opposite end of the spectrum (Pacus, silver dollars). None the less do offer greens to your piranhas and some animal-based foods to their less toothy relatives daily. 

Disease, Prevention/Cure

    When stressed, particularly through rapid temperature change, especially drops, Serrasalmine fishes can contract parasitic diseases (e.g. ich, velvet) quickly. Small individuals of fishes can succumb to these in a short while, even less than a day. Keep their environments thermally stable.

    Treatments for parasitic disease need to be carefully monitored, as these fishes are sensitive to the typical dyes and metal salt medications used in the ornamental aquatic interest. Take care to know the actual gallonage (not the "model" gallonage) of your system and don't over-medicate.

    Infectious disease (bacterial, fungal) are almost always related to water quality issues and are remedied by improving same, not by simply administering anti-microbials.

Cloze:

    Are you surprised at how many members of this subfamily there are? At the number of piranha? At the size of big Pacus? All of these fishes can be kept (if they can be found, and are legal in your area!) in larger tropical freshwater systems. If you have the chance, check out the Public Aquarium exhibits displaying these fishes, their biotopes. The ones in the Steinhart (San Francisco), Baltimore, Singapore and the Wilhelma in Germany are amongst my favorites. Thousands of gallons of planted, large sunken trees, subdued lighting, schools of various Serrasalmine fishes along with cichlids, catfishes, and smaller tetras coming from the same regions. 

Bibliography/Further Reading: 

Brandy, George and Douglas Campbell. 1984. Some notes on spawning and rearing the Red-Bellied Piranha. FAMA 7/84.

Dunker, Toni. 1960. Catoprion mento, the Wimple Piranha. TFH 1/60.

Fenner, Bob. 1993. An argument against "Feeder" Goldfish. FAMA 11/93.

Jennings, Ron. 1978. How to live with the Red Piranha. FAMA 6/78.

Meegaskumbura, Madhava P.B. 1999. Breeding and caring for Silver Dollars. TFH 9/99. 

Neal, Tom. The Pacu- A friendly giant. TFH 8/98.

Nico, Leo G. and Donald C. Taphorn. 1986. Those bitin' fish from South America. TFH 2/86.

Quinn, John R. 1992. Piranhas. Fact and Fiction. T.F.H. Publications, NJ. 128pp.

Schleser, David M. 1999. Piranhas. A bum rap. Finding the truth about the misconceptions. AFM 3/99.

Schultz, Harald. 1960. Piranhas- Fact and fiction. TFH 9/60.

Vorderwinkler, William. 1960. The Piranha- a menace? TFH 2/60.

Walker, Braz. 1970. The colossal creampuff. The Aquarium 1&7/70.