In their natural environment fish live in a massive volume of water. It can take days for weather to alter the temperature of the water. Organic matter decomposes and produces ammonia and nitrites which can be harmful but in a large body of water they are diluted to a point where its impossible. Aquariums however are the opposite. Tiny changes in room temperature can affect their entire environment. Ammonia and nitrites have to be managed and most municipal water is treated with chlorine or chloramines which are put in to kill living things.
Aquariums need three types of filtration to maintain a healthy environment for freshwater, saltwater or amphibian aquariums. First is sediment filtration, removing waste, uneaten food and debris from the water. The second is carbon filtration using activated carbon, such as GAC cartridges, to reduce or remove chemical pollutants. The third is the removal of biological contaminants, such as ammonia or nitrites.
Your specific filtration needs will vary depending on the type of water you have and what may be in it, the type of aquarium (freshwater, saltwater, amphibian) so consulting an expert is recommended. Forums like The Reef Tank and Reef Central can offer some expert advice.
Reverse osmosis water filtration systems can remove 90% of total dissolved solids (TDS) from water and can provide the purest water available for your home. Reverse osmosis water filters remove a wide range of contaminants and minerals from your drinking water, including sand, chlorine, fluoride, cryptosporidium, and hexavalent chromium.
AF Deionization cartridges provide solutions for variety of applications that need the removal of ionic contaminants.