Water chemistry
Oxygen
Aquatic plants, animals, and bacteria depend on oxygen dissolved in water for respiration. Oxygen can be added to the aquarium through an air pump attached to some sort of air stone, by utilizing a filter that creates much surface disturbance, or by using a wet-dry filter system. Plants also add oxygen during the day with photosynthesis, although use oxygen at night during respiration. Thus, in a planted tank, the oxygen levels fall at night. At a higher temperature, less oxygen is dissolved in water and more aeration is required.
Carbon Dioxide
Carbon dioxide is another gas present in water as a byproduct of the respiration of organisms in the aquarium. During photosynthesis, plants require carbon dioxide. When there is a lack of carbon dioxide, the leaves of plants begin to yellow, and growth slows. Some of the carbon dioxide dissolved in water forms carbonic acid, which lowers the pH. If there is too much carbon dioxide in the tank, the fish will suffer.
Adding Carbon Dioxide: Carbon dioxide can be added by lower the water hardness to free CO2 from calcium bicarbonate. CO2 can also be added by the use of a CO2 system.
The Nitrogen Cycle
When fishes excrete waste, plants deteriorate, and food rots, the resulting waste does not just disappear. The waste, in the form of nitrogen compounds, is broken down into other compounds by bacteria, until the final product, nitrate NO3 is absorbed by plants or removed through gravel siphoning. This process of converting NH3/NH4 (of fish, plant, and other waste) into NO3 is known as nitrification. The steps of this process are most evident when the tank is first established or when the tank is out of balance.
When the tank is first set up, there are not nitrifying bacteria present (unless gravel or filters from an already established tank are used). A few hardy fish are added to the tank. By the first day, trace amounts of ammonia/ammonium (more toxic ammonia if the pH is above 7.0, less toxic ammonium when the pH is below 7.0) have built up. Within a few days, the ammonia level rises to a toxic level. By this time Nitrosomonas bacteria have begun to develop. These bacteria begin to break down the ammonia into nitrite (NO2), which is less toxic than ammonia/ammonium. Another type of bacteria, Nitrobacters, break down the nitrite (NO2) into less toxic nitrate (NO3). The nitrate is absorbed by plants or algae, or is removed when a water change is carried out. Although nitrate is less toxic than other nitrogen compounds, in high levels, it can be toxic.
This cycle also occurs when the tank becomes out of balance. If waste levels become too high, because of over-feeding, overstocking, and/or lack of water changes, ammonia levels will rise again. In this case, water changes should be performed until normal levels are restored.
This cycle can occur when the bacteria that breakdown organic wastes die. This can occur when the tank or filter is washed with soap or some disinfectant. If this occurs, the tank will have to be cycled again. Be aware that too acidic (below 5.5 pH) water or some medications can also deplete the bacteria population.
Nitrosomonas Nitrobacters
NH3 —————–> NO2 —————–> NO3
excess food absorbed by plants
fish waste removed during water changes plant matter
death
