All animals - and fishes are animals in the same sense that birds, mammals, insects, and worms are animals, in that they're not plants or minerals - are categorized into different types. The animals are grouped according to likeness and differences that appeal to the people who do the classifying as being sensible points of departure. The points of departure are physical characteristics that usually are easily observable and therefore verifiable in terms of their presence or absence: a bird has feathers, but a mammal doesn't; an insect has legs, but a snail doesn't; these three fishes have scales, but that one doesn't.
It is assumed by the classifiers that all of the animals are more or less related, and it is further assumed that the points of departure have significance in determining the degree of closeness of relationship. The classification of animals according to their supposed relationships is called taxonomy, and the people who do the classifying are called taxonomists or systematists. Their work is vital to many other areas of biological study, if for no other reason than that it facilitates communication by providing generally agreed-upon identifications: you can't tell the players without a scorecard. Where animals are concerned, the taxonomists amke the scorecards, and their tools are the taxa (categories) they devise.
Like all other animals, fishes are classified into big, very inclusive groups that are broken down into smaller, less inclusive groups, with the smaller groups being broken down into yet smaller and less inclusive groups. Ranged in the order of descending inclusion, for example, for fishes the biggest and most inclusive group would be the kingdom Animalia, which contains all animals. Then would come the phylum; for fishes it would be the phylum Vertebrata, which contains all animals with backbones. Less inclusive would be the next taxon, the class, which for aquarium fishes would be the class Osteichthyes, the bony fishes (in cartilaginous fishes, the sharks and rays). Continuing in descending order of inclusiveness, the remaining taxa would be the order, the family, the genus and the species. Things aren't always as clear-cut as the taxonomists would like them to be, so in certain areas they've had to set up intermediate taxa in the form of superorders and subfamilies, subgenera and subspecies and suchlike to make distinctions between each group clearer.
The four taxa that we'll be concerned with in this book are the order, the family, the genus and the species. We'll be concerned with them in only the most elemental sense, becasue all we'll be doing is just throwing the names around. But still you'll be able to see from the photos in the book what the taxonomists are basically driving at: the fishes in a given family look as if they are related; they at least generally look more like eaxch other than they look like fishes from other families.
Five orders (the Cypriniformes, the Characiformes, the Siluriformes, the Cyprino-dontiformes and the Perciformes) comprise probably 95% of the fishes in the freshwater aquarium hobby. The remaining 5% are spread over another four or five orders. Just for the record, here are four orders that contain the bulk of aquarium fishes, with the listings of the best-known families of aquarium fishes that each order contains:
Characiformes contains all of the many small "tetras" such as the cardinal tetra, the neon tetra and the lemon tetra of the family Characidae, (which includes, among others, the subfamily Serrasalminae (the piranhas), hatchetfishes (family Gasteropelicidae), and the pencilfishes (family Lebiasiniadae).
Cypriniformes includes all of the carp-like fishes such as the barbs, rasboras and danios, as well as (again among others) the loaches.
Siluriformes contains numerous families of catfishes, including a great diversity of aquarium fishes.
Cyprinodontiformes contains the livebearers as well as the killifishes and a few other families.
Perciformes contains the cichilids (such as the angelfish) and the anabantoids (such as all of the gouramies and the Siamese fighting fish); it also contains the spiny eels.
Most families contain a number of genera, but some families have only one. Likewise, most genera contain more than one species, but some contain only one. It's possible, then, for a family to contain only one species. That's an unusual situation, but it can occur; among aquarium fishes, an example of a family containing only one species is the family Badidae, which contains Badis badis, the chameleon fish.
You'll have noticed from even the very limited amount of taxonomy presented here that there are certain conventions observed in presenting the names of the different taxa. The names of all of the orders, for instance, end in - iformes, the suborders in - ei and the families in - idae. Also, the names of genera and species are given in italic type, whereas all the taxa above the genus are kept in regular type. Aditionally, the first letter of a specific name is not, even if the specific name is coined from a proper name, such as a person's name or a geographical place name.
In effect, the generic and specific names applied to a particular fish are the name of the fish. Some tropical fish hobbyists take pleasure from learning and using the names of not only the fishes they own but also the ones they've only read about or seen in dealer's tanks. They may mangle the pronunciation of the names and may have no idea what they mean, but they have at least a partial grip on them. Other hobbyists deliberately ignore the scientific names, insisting on being provided with "common" names instead. I think the people in the latter group are missing out on a nice little slice of the hobby's charm, and they're giving it up for no good reason. The names aren't that tough to master, as witness the fact that some who feel queasy about spitting out an occassional piscine generic name will blithely tell you how their chrysanthemums and rhododendrons are doing.
The fishes get their names, incidentally, from the taxonomists, who put names on fishes that they think have never been described and named before. Certain internationally agreed-upon rules and a certain etiquette govern the publication of scientific papers naming new species, but essentially it's a process of describing what is hoped to be a new-to-science fish and showing how it differs from known species it's supposed to be most closely related to. You'll often see the name of the fish's describer (it's "author") following the name of the fish when the fish's name is referred to in print; often the year of the description is also listed. Sometimes the describer's name is set within parentheses; the appearance of the parentheses signifies that the generic name given for the fish is not the one originally applied by the person whose name appears within those parentheses. A plain "L" or "L". after a name is an abbreviation for Linnaeus, which is the Latinized version of the surname of Karl von Linne, the 18th century biologist who was most instrumental in hyaving the system of coupling a generic and specific word-pair accepted as the name by which naturalists would recognize a particular animal. The system has worked well for over 200 years now.