Oxygenation
Carbon dioxide is removed and oxygen is replenished only at the water surface. Water movement at the surface of the aquarium achieves this. Surface agitation breaks the surface tension and brings more water molecules into contact with the air. Oxygen can only enter the water, and carbon dioxide by released, at the interface between atmosphere and water surface. The greater the water surface. The greater the agitation (within reason), the more efficient the exchange and the higher saturation of oxygen. Although air stones do break surface water tension, they are much less efficient and effective at oxygenation than power filters which generally move and expose a much larger water-to-air ratio.
Water Chemistry Tips
Water is the single most important ingredient in an aquarium. If it is polluted or toxic, success is rare. Never use a bucket that has had soap or detergent in it. Plastic retains traces of soap, which can pollute the water. Obtain a bucket, label FOR FISH USE ONLY, and only use it for the aquarium. Keep it separate for your aquarium. The largest killer in new aquariums is toxic water. Most municipalities treat their water supply with chemicals to make it safe for human consumption. The most common chemical added is Chlorine. It is deadly to fish and must be removed if fish are to survive. Chloramine is sometimes used instead of Chlorine, which more difficult to remove.
The Nitrogen Cycle
Fish, as they live and breathe, create ammonia and send it into the water. Rotting food, fish wastes and decaying plant and animal matter also add to the ammonia production. In natural situations, the surface area of the water body is so large that ammonia naturally dissipates to the atmosphere or can never reach dangerous concentrations because of the tremendous water volume. Some of it is used as an energy source by the rest of the aquatic community. In the sterile environment of the new aquarium, there is no "rest of the aquatic community". The surface area is so limited that the ammonia will tend to concentrate rather than dissipate to the atmosphere. Normally, it takes quite a bit of time to culture the bacteria that comprise the "hidden filter" that must remove ammonia from the environment to make the aquarium habitable. Ammonia becomes part of the water solution; it cannot be removed by normal mechanical filtration methods. Instead there are numerous bacteria that uses ammonia as a food source. The important strains for the Nitrogen Cycle are the lithotropic bacteria nitorsomonas and nitrobacter. They are stationary bacteria that attach to clean rocks, glass, and wherever there is a good attachment node for the colony to begin. Since they are aerobic they require free access to oxygen and a good flow of water to make the food source available. Both types of bacteria are very slow in replication compared to most other bacteria, so it can take a long time to get a strong population of the desired strains. The biological filter grows almost anywhere the conditions permit, however it is strongest where strong oxidation and abundant nutrients are easily available. Power filters easily provide both vital requirements with the large filter volume of installed inserts supporting colonies of bacteria. Ask yours sales associate to recommend a test kit to help solve your water chemistry concerns.