Assembly and Decoration
With an appropriate recipient and water we already have a tank.
Even so, to describe that item, I will base on a tank of glass, rectangular, with capacity for about 50 liters of water, and, initially, as simple as possible in relation to the equipment. The tank should be clean and without residues of chemical products of cleaning.
The first step is to place the aquarium in the definitive place, where it should be well fixed, without forming any angle in relation to the soil. 
Then It is placed the gravel, previously washed with water, until a height of approximately 5 cm, and on the gravel the decoration of rocks and logs. Then we fill with water up to 2/3 of the height. As the water of our faucets contains a great amount of chlorine that is harmful for the plants and also for the fish, the water must be treated with some product that removes chlorine, found at fish stores, or the water should be left resting for 2 days so that the chlorine evaporates before planting the aquarium. The reason of filling 2/3 of water is just not to overflow the aquarium when we place the hands to plant. The plants should be buried in the gravel with delicacy, taking care for not hurting the roots (if existent); green leaves should not be buried, they should be removed of the part that will be buried so that they don't come to rot under the gravel. Now finish filling the tank, not forgetting to remove the chlorine of the water before. After that step, the aquarium should be left in rest by some days (one week or more) so that a rudimentary biological cycle begins (since any kinf of filter was not placed in the aquarium to stimulate the cycle) and the plants begin to create roots that will fasten them better in the soil. In that phase, many plants must be replanted, when they loose the gravel and float. It is common that after 2 or 3 days of set up the water of the aquarium becomes milky, what is a sign that the fishbowl is beginning to form its biology.
Passed the period of fixation of the plants, and when the water be crystalline again, the introduction of the fishes can begin. It is advisable to place few fishes each time, leaving some days pass before adding more fishes, so the biology can adapt to the demands of the fishes. We must never put fishes in excess in the tank (the tank can house just a limited number of fishes) because those can suffer for lack of oxygen or be intoxicated by its dejections that are accumulating in the botton, before decomposing.
It is now that we introduce the filters… (and the occasional water changes).
The biological filter is traditionally a group of perforated plates placed in the botton of the tank, below the gravel layer, that has for function to facilitate the circulation of water in the gravel layer, providing oxygen for the important bacterias that fix on the gravel. 
To force the circulation of the water throught the biological filter a pump is used, and it can be put inside or outside the tank (there are the internal and the external kinds of pumps). As the plates of the biological filter are going underneath the gravel, those should be placed soon in the aquarium in the set up, before the placement of the gravel. An aquarium with a stabilized biological filter allows that more fishes are housed per liter of water, because besides transforming the dejections of the fishes in less toxic products, it facilitates the change of gases in the surface of the water, through the movement.
With relationship to external filters, there are of several kinds. In my opinion, with the modern technology and the lowest prices, I advise to aquire a filter that has the pump already acopled, and that uses pads that can be changed (discarded and substituted) when the filter elements get old and less efective. Actually there are kinds of external filters with sponges to shelter the layer of bacterias of the biological filter, not being necessary in that case the use of the plates below the gravel. The bacterias will fix in this sponge, and the amount of bacterias that can fix may suffice to the needs of the aquarium; as the water will flow all the time and in great amount throught this sponge, the bacterias will receive a lot of oxigen all the time, and the biological filtration will work in full capacity.
Besides the filters, regular water changes also help to maintain the aquarium clean and safe, since when we change water we use to remove part of the dejections that accumulate in the botton of the tank. The amount of water changes will depend on the amount of fish and on the filters used. The water used in the water change cannot contain chlorine, so it must be treated, or rest for 2 days before be placed in the aquarium.
An important detail to be mentioned is the illumination of the tank, since the plants need light to develop. I particularly find the illumination a more important item than the own filtration. It can be find at fish stores gutters that are ready to be used in the aquarium, and in different sizes, so it can be fitted to any size of aquarium and lamp that we need to use. There are special kinds of lamps for the illumination of ornamental aquariums, with a light spectrum more adapted to stimulate the growth of the plants, to show the coloring of the fishes better, and so on. I consider that the lamps Aqua-Glo are good enough to begin. Even so, as we can choice not to use natural plants and depending on the amount of light that the aquarium receives by its location in the place, not always it is necessary to use special illumination. In relation to the tanks without plants, it is even possible not to use gravel in the botton, leaving just water, or to placing some decoration in the botton, as a log, or a broken vase of mud. It is also common to find aquariums with artificial plants in the place of the natural ones. This plants can look almost like the natural ones, but they don't demand specific cares.
Doubts? Suggestions...
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