These inhabit various biotopes in tropical and equatorial regions. Basically they may be divided into two groups. The first comprises those for which soft, acidic conditions are essential. Some require a very low level of hardness, with a typical pH of around 6. The second group prefers hard, alkaline conditions. For some species the level of hardness must be extremely high, with a pH of up to 8. A few species can survive in brackish water. There are some fish which are not dependent on water quality. These are consequently ideal for the beginner, who can, in principle, fill the aquarium from the household supply. Though there are over 10,000 species native to inland waters, only a few hundred need concern the aquarist.

There are almost 20,000 species of marine fish in the world, but only a small minority is of any interest to the aquarist. Of these, most come from the Pacific and Indian oceans, although a few are native to Australia and the tropical Atlantic ocean. Marine fish are usually collected in their natural environment and rarely reproduce in captivity. They find it more difficult to acclimatize than freshwater fish, particularly as regards feeding: some refuse to eat artificial food, and others have very special dietary requirements. The smallest marine fish require a minimum water volume of 150-200 liters. Medium-sized species (around 20 cm) must he kept in tanks of 300 liters, or at least 400 liters if they are active. It is advisable to obtain juvenile (or sub-adult) specimens, as they are generally easier to acclimatize and feed.

These days, more and more aquarists seem to be taking an interest in these animals. Ecologically speaking, invertebrates represent a natural complement to fish and plants; in visual terms, you can put on a spectacle of luminous beauty, particularly in sea water, using anemones and corals, to which can be added small fish, either lively or placid, but always brightly colored. There are also a few species of freshwater mollusks and crustaceans which are easy to keep in captivity, although they are little known and often overlooked.

Of the thousands of plants that are genuinely aquatic, or only amphibian, only a few hundred are to be found in the aquarium trade. The vast majority of these come from tropical fresh water, with the remainder coming from northern climes, and therefore suitable for temperate aquariums and garden ponds. The most common plants are generally the most robust, and are therefore specially suitable for beginners. Others are best left to committed aquarists keen to create a veritable aquatic garden. The special plants - the floating species, mosses, and ferns - have been grouped together, on account of their exceptional lifestyle; they should be of interest to all aquarists.