What is a "Shark"?
"Shark," as it turns out, is a very generic title when applied to freshwater fish. Most of the freshwater fish called "sharks" are actually bony fish and have no close relationship to the cartilaginous carnivores of the oceans, but some of the fish having the "shark" epithet do have characteristics reminiscent of their saltwater namesakes. For example, the Shark Catfish, Sciades seemanni,has been known, on occasion, to grow over two feet long and can quickly gobble up smaller tankmates. Also, contrary to the fact that it is often sold as a freshwater species, the Shark Catfish requires a marine/brackish environment as an adult. Despite that some of the freshwater species called "shark" can be rather pugnacious, it is often the case that the title "shark" applies more to body shape than it does to temperament. As a case in point, the Rainbow Shark, Epalzeorhynchos frenatus (pictured at left, courtesy of Fishchannel.com), has a vaguely shark-like body due to a prominent, pointed dorsal fin and a deeply indented caudal fin. Since the title of "shark" is so widespread in the realm of freshwater fish, simply calling oneself a freshwater shark enthusiast is not very descriptive, but most aquarists who do describe themselves in this way are talking about a particular group of cyprinid fishes related to barbs, danios, and rasboras. This particular group is often seen as being problematic, as even difficult to keep. Personally, I prefer to think of these sharks as being more misunderstood than problematic.
The main sharks in the cyprinid group of fishes are the Golden Shark (Leptobarbus hoevenii), Bala Shark (Balantiocheilus melanopterus), Banded Hi-Fin Shark (Myxocyprinus asiaticus), Black Shark (Labeo chrysophekadion), Red-Tail Shark (Epalzeorhynchus bicolor), and Rainbow Shark (Epalzeorhynchos frenatus). My area of expertise with regard to sharks are Red-Tails and Rainbows, but based on my experience selling the other species, I offer some general comments for anyone thinking about keeping Golden, Bala, Hi-Fin, or Black Sharks:
Red-Tails and Rainbows: Territorial-Hierarchical
Now, as to my favorite shark species, Red-Tail and Rainbow sharks are often sold and kept according to a simple guideline: one shark per tank and make sure your fish can handle these ruffians. This rule generally works, but is far short of really helping aquarists understand how to make the most of their sharks. Furthermore, few aquarists actually try to understand shark behavior; many aquarists label sharks as semi-aggressive and do not give them a second thought. By contrast, very few serious aquarists would hesitate to say that cichlid behavior is complex and deserves considerable attention on the part of the hobbyist. Sharks have a behavioral set just as, if not more, interesting than cichlids, and an understanding of that behavior can be a considerable asset to the aquarist who wishes to try interesting fish combinations.
Before discussing sharks in great detail, I find it helpful to first remind aquarists of the behavior pattern of a fish that, though only distantly related to Red-Tails and Rainbows, has traits that are indeed similar. This similar fish is the famous (as well as infamous) Tiger Barb. Tiger Barbs are colorful fish that are relatively hardy, but they also have a reputation for being fin-nippers and can harass other fish to death (fairly frequently, actually). Even though Tiger Barbs can be so problematic, there are a great number of aquarists who not only prefer to keep them but also keep them successfully with fish that, under ordinary circumstances, would be damaged by the Tiger Barbs (i.e., Angelfish, Pictus catfish, etc). How do these aquarists accomplish such feats? Simple, these select aquarists understand that Tiger Barbs are a Hierarchical-Shoaling species.
As fish behavior is concerned, hierarchical means that the fish species has and regularly maintains a social pecking order. This means that Tiger Barbs, which are Hierarchical-Shoaling, are pre-programed to keep each other in check, test each other's power, and spend a lot of time engaged in such activities. This facet of being Hierarchical is what makes Tiger Barbs potentially pugnacious toward other fish species. For the barbs such confrontations are perfectly normal, but many other fish simply do not understand the barb's behavior--the price of this miscommunication is often severe for the other fish. Furthermore, shoaling generally means that the fish species has what one may call group-identity, but not necessarily group-dependency (which would be denoted by the term Schooling). In other words, Tiger Barbs intentionally associate with each other and prefer the company of their owns species to that of another species or solitude. This affinity for their own kind is what allows aquarists to control their less admirable qualities.
How do aquarists control Tiger Barbs? Tiger Barb aggression can be kept in check using two basic techniques: 1) keep them in a very large shoal (as in more than 10 individuals preferably, 6 minimum) and 2) separate this shoal from the other fish by placing them in a sufficiently large tank (at least 30 gallons, 55+ gallons being optimal). Doing these two things satisfy the Tiger Barb's basic behavioral needs by giving them plenty of targets for their hierarchical endeavors that understand what is happening and give the other fish in the tank a chance to remove themselves from the turmoil. Now, Red-Tails and Rainbows are very similar to Tiger Barbs in that shoals and tank size can help defray aggression, but there is an important addition to the list of things that will help aquarists keep sharks successfully: structure.
Unlike Tiger Barbs, Red-Tails and Rainbows tend to be territory-holders in nature and identify their territories by structures that are in the tank. That is, instead of shoaling or schooling behavior which allows groups of fish to migrate almost aimlessly about the aquarium with disregard for where they are in the tank specifically, Red-Tails and Rainbows tend to reference their movement by nearby structures rather than by the fish that are or are not near them. This identification of one's space with specific structures is called territoriality and, thusly, Red-Tails and Rainbows, as they have a social pecking order but base their movements on structures instead of other fish, are territorial-hierarchical fish.
The consequence of this difference between fish like Tiger barbs, which base their actions on each other, and sharks, which are structure-based, is that what allows an aquarist to be successful with one group may not work for another group. In the case of sharks, placing a large shoal in a large tank, as the only provisions, will not be successful in most cases as the individual fish will have no reference points and thus perceive the entire tank as their individual territories. The solution is to arrange the tank so that each individual has its own reference points that do not overlap with another fish's reference points. In other words, when a shark is in its territory, it should not have to feel like it is sharing the space with another shark. The net result is that aquarists who wish to keep multiple sharks need structures in the aquarium that separate the tank into distinct zones from which each shark cannot easily see other sharks.
As distinct zones are a basic requirement for keeping sharks, there have evolved two basic tank decorating schemes that respond to this need. The first is that of back-stuffing the aquarium, or placing a myriad of decorations along the back wall of the aquarium such that the back third or half of the aquarium is occupied (from top to bottom) with crevices, nooks, crannies, etc. This density prevents the fish that enter this back portion from seeing each other. This method is fairly useful in planted tanks in that you can simply keep the vegetation dense in the back and then leave an open area in the front of the tank for viewing the fish. Also, setups that are frequently used for African cichlids fall into this category of back-stuffing in that such setups are comprised of a rock wall along the back that offers the fish their own little hiding spots.
The above method works well but it also means that the aquarist is not likely to see the fish very often in that they will spend a lot of time in the back decorations hiding from the other fish. This being the case, another zoning scheme that can be considered is compartmentalization. Compartmentalization involves breaking an aquarium into separate compartments (one compartment per shark) by using decorations that are almost as wide as the aquarium and almost as tall as the aquarium arranged from front to back (items are arranged not from left wall to right wall along the back but vertically, from front to back). This system breaks the aquarium into mini-aquariums that are relatively isolated from the other subsections and, as fish either hide to the left or right of the structures, the fish remain in view of the aquarist more often.
One last detail should be mentioned about sharks: they are very slow to warm up to a new home. People who want to see their sharks out and about all the time will be disappointed in that sharks are fish that like their structures. Their structures make them feel secure and it is not until sharks are very familiar and happy with their new home that they begin to venture into the open fairly often. This acclimation period can take a few weeks.