One thing that sets fishkeeping apart from other pet keeping hobbies is its unique environment. Dogs, cats, birds, reptiles, rabbits, hamsters, and other pets live on land and basically live the same way as we humans do. Just provide the same environment as ours and they will be generally fine. For example, a birdcage inside your house should provide ample environment that is livable enough for your pet bird.
Since fishes live in water, you will have to recreate this inside the confines of an aquarium. It is not merely keeping water in the tank, but also providing a filter system that sustains good water quality. Unfortunately, fishes do not have bathrooms where they can defecate and urinate as we humans do. They just release their biological wastes in the very same water they live in. Fishes are highly dependent on the quality of water they live in, so if the water is polluted, they can get sick or, worse, die.
The aquarium is really very restrictive. Considering that fishes live in vast rivers, lakes, and seas, an aquarium, no matter how big, is minuscule compared to a fish’s natural environment. With limited space, aquarium water will quickly get polluted once the fish produces waste.
Unfortunately, setting up a filter system in your aquarium will not automatically effectively and efficiently clean the water. Your filter will have to be biologically active for it to function correctly. This is where the nitrogen cycle comes in.