The Ecology of the Ornamental Aquarium
An ecosystem is a closed system, that is self-sufficient and is maintained by the interaction between its components.
The fishbowl doesn't work exactly as a closed ecological system, because it is necessary to introduce food, to do a periodic cleaning of the botton, eventually to change water and possibly to use filters, and maybe heating up the water artificially. Even so, one of the the objectives of the hobbyst is to reach a balance of the system.
A ready tank, with biological filter under the gravel layer, some plants and fishes, presents cycles of decomposition. Some elements are decomposed and transformed in other elements, that may be used by the plants, or just be less toxic than the first elements. For example, there are the oxygen cycle and the biological cycle, which reach a balance in a stabilized aquarium.
In the case of the biological system, the digestion of the food by the fishes introduces ammonia in the water (which is toxic), that induces the fixation of bacterias in the gravel layer, that decomposes the ammonia, transforming it in nitrite (which is also toxic), what will stimulate a new kind of bacteria to develop, that for its time will transform the nitrite in nitrate (which is non-toxic). The plants in the tank absorb some nitrate in the water, but in very small amounts; and, to close the "cycle", there are some species of fish that feed on plants, and that will be eaten by the carnivorous fishes. As the use of the nitrate is much smaller than its production, it will accumulat in the water, until a water change is done. One of the functions of the water changes is the removal of the excess of nitrate. It is fact that a great concentration of nitrate influences the plants, decreasing its grouth, and it is speculated that it is also harmful to the fish, especially in the salt water tanks, that tolerates smaller concentrations of nitrate.
The development of the biological filter is the first step to keep plants and fishes healthy. This is done when the different kinds of bacterias of this cycle fix in the gravel layer, and begin decomposing the elements. The time period for the complete stabilization of the biological filter is approximately one month (see graphic). It is advisable to not place fishes in the aquarium before this period. I like, particularly, to put, after a week of set up of the tank, just one fish that will produce some ammonia and stimulate the formation of the biological filter, producing this initial element. After the first month it is advisable that the fishes are introduced slowly, not all in the same day so that the biological filter adapts the demand of its function. Every tank has a capability to house fishes, in other words, the tank can house a limited number of fishes, and if we put more fishes than the tank can house we will have problems: if we put excess of fishes, the balance of the tank will became vulnerable, being affected easily. For example, a fish that dies and begins to decompose in an empty tank doesn't cause any larger upset, but in a full tank it can cause the death of all the other fishes if it is not removed quickly, because the decomposition uses great amounts of oxygen, that will be missing for the other fishes.
The oxygen is mainly dissolved in the water through changes with the surface, but it is also produced by the aquatic plants through the photosynthesis. The more hot the water, smaller the amount of dissolved oxygen that it will contain. Without heating system, the water of the tank warms up during the day and it cools during the night, so there are more oxygen in the water during the night. That is the reason to avoid to place a lot of fishes in a fishbowl (more than it can house) and also to use aeration or other way to move the water, to stimulate the gas exchange: the exit of the carbonic gas and entrance of oxygen, through a dynamic balance.
This brief introduction shows why so many tanks donīt work, and the hobbyst loses its interest for the hobby. It is easy to notice also that it is very important to have some patience when setting up a new aquarium, so that it becomes stabilized before the inhabitants' placement. It is common to see people (especially accompanied of children) in the fish stores, buying a complete KIT: the fishbowl, filters, plants, and fishes, everything at the same time. The understanding that it is necessary to prepare the habitat in advance, for the later introduction of the fish, is fundamental for the growth of the number of hobbysts.
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