Ever brought home a fish only to have it die within a few days? If so, your finned pet likely fell victim to "new tank syndrome" - an unfortunate situation new fishkeepers often get into, but one that is also very easy to prevent.
FACT - The natural lifespan of the common goldfish (Carassius auratus) actually ranges from 10-35 years! A far cry from the few days or weeks that they often last in home aquariums. These "beginner" fish are regular victims of new tank syndrome, to the point that a myth has developed about their "short" lifespans!
In nature, bodies of water are part of complex ecosystems involving the earth that the water sits on and the microorganisms and plants that inhabit it. This ecosystem works to break down and utilize the waste products of the fish that inhabit the water.
This is in stark contrast to the closed, sterile environment of a new aquarium. The tank and all equipment in it are clean and house relatively none of the bacteria in nature that would break down fish wastes. Nor does the water added to the tank contain these beneficial bacteria, as chlorine and chloramines are added to tap water in order to kill off bacteria/purify it for human consumption. The addition of these chemicals is why the use of a water conditioner which addresses both of them is necessary (such as Seachem Prime). Please see the BETTA MYTHS section for a misconception regarding water conditioner and "aged" water.
Fish secrete ammonia into the water column as a metabolic waste product. Also, as solid wastes, such as fish feces or uneaten food, break down, they produce ammonia. If allowed to remain and/or build up in the water column, ammonia is toxic to fish, causing burns and damage to their gills and eventually death. Remember that in an aquatic environment, fish are breathing their own waste.
To make matters worse, the standard filter cartridge that most filters come with contains activated carbon - a chemical filter media that has no ammonia removing properties. Such a filter would not be able to remove harmful waste products from the water, and thus protect fish, until it had gone through the "nitrogen cycle" (ammonia and nitrite consuming bacterial colonies had grown and established themselves on the filter sufficient to process the waste output of the fish). This cycling process can take 30 days or more, and ammonia can build up to toxic levels in a matter of hours.