Before you begin the wonderful hobby of fish keeping it is advisable to learn something about the naming and classification of fishes. This article explains the fishes for your aquarium.
Among fishes there are many which have no English name, though as they become popular they tend to acquire them. Generally speaking, however, it is better to learn the scientific names, because these are internationally used, and more exact.
These names are usually derived from Greek or Latin words, often descriptive of some feature of the fish concerned. They are always in two parts. The first part, beginning with a capital letter, is called the generic name, and indicates the group to which the species belongs; the second part, always beginning with a small letter, is the trivial name, indicating the particular species.
More than twenty thousand kinds of fishes have been discovered, of which about a third come from fresh waters; new ones are being found every year.
This vast assemblage is classified by scientists according to their relationships as shown by careful study of their structure and way of life. The genera, or groups of related species, are put together in families (groups of related genera), and these into orders.
Broadly speaking the more primitive kinds of fishes have only one dorsal fin, mostly with soft rays, and often an adipose fin, while the pelvic fins have no spines; the more advanced fishes (those which evolved more recently) have the dorsal fin in two parts, the front part bearing spines, and have always a spine in front of each pelvic fin, and usually two or more in front of the anal fin.
Only a mere handful of the known species - about 200 - are familiar to the aquarist, and for him there is a much more useful classification. Fishes can be divided into tropical and cold-water species.
The tropical fishes can be divided again mostly according to the way they breed. First, there are the livebearers, which give birth to young ones much as higher animals do; secondly there are those that lay eggs and take no further interest in them - perhaps even eating them as soon as they are laid; thirdly, we have those which lay eggs but take care of them, and these can be again divided into those that make a nest of bubbles at the surface, and those that protect them in other ways.
The name Livebearers refers to the fact that the eggs hatch out within the body of the female, and the young fish are not liberated into the water until they are well-developed enough to swim about and feed.
This means that the eggs must be fertilized internally, and for this purpose the male fish is provided with a special structure known as a gonopodium, formed by the front part of the anal fin. The sexes are therefore easy to distinguish in these fishes, and the veriest novice can be sure of getting a pair. The female is often larger than the male.
Another remarkable thing about them is that a single fertilization enables the female to produce two or three, or even more, broods of young, so that it is possible to purchase just one healthy, plump female and soon have a thriving family. Most females, in fact, will be found to be in this condition, and can easily be recognized by a dark patch showing through the flesh at the end of the body cavity, known as the "gravid spot".
The broods are produced usually at six- to eight-week intervals, and average about 25 young. These grow quickly and become mature after two or three months, but they are still small at this time, and if allowed to breed will not grow much larger.
Goldfish are the most popular of the cold-water fishes.
Thus it can be seen that there is indeed a wealth of fishes which may be bought to stock your aquarium. In no time at all you will have a tank which will entertain you for hours.