Nitrogen Cycle
Perhaps the most important factor of healthy fish and successful fish keeping is the Nitrogen Cycle. The nitrogen cycle involves three main types of bacteria, two which are aerobic (need oxygen) and one, which is anaerobic (does not need oxygen).
The first type of bacteria, Nitrosomas, get their energy from oxidizing ammonia NH3 (ammonium NH4+ in water) into Nitrite NO2-. The second form of bacteria, Nitrospira, get their energy from oxidizing nitrite into nitrate NO3-. Now ammonia and nitrite are very toxic to fish and the build up of even minute amounts of these toxins can have lethal effects. Thus possibly the most important bacteria is the one that converts the nitrites into much less toxic nitrates.
Now in a nutshell, this is the nitrogen cycle in aquaria where the nitrates have to be diluted through water changes; however this is not the complete nitrogen cycle because in nature, anaerobic bacteria take the nitrates and reduce them to nitrites then to ammonia and finally to nitrogen gas. Then some types of bacteria and other life forms, like cyanobacteria, fix the atmospheric nitrogen (take the nitrogen gas and incorporate it into body mass). These bacteria and/or life forms would eventually get consumed by other organisms in the food chain and eventually get eaten by fish to start the cycle all over again.
In new aquarium set ups, the two forms of aerobic bacteria are not yet present and need time to build up their colonies, thus ammonia which is excreted by fish, mostly through their gills as a waste product, cannot be converted to nitrite and then to the much less toxic nitrate. This is where most fish keepers encounter problems with keeping fish alive and get discouraged and leave the hobby altogether.
pH
pH indicates the concentration of acids and bases present in a solution. PH is primarily affected in aquaria by the amount of carbonic acid and the degree of carbonate hardness present in the water. Other substances such as humic acids also play a role in pH. The pH range is between 0 and 14. Below 7 is acidic and above is alkaline.
Many fish are used to a specific pH as found in their native habitats. Although these days, many fish species are used to a similar pH (pH 7.0 usually) because they are bred in captivity. So the more crucial to fish health is not so much the level of pH rather any changes in pH, especially rapid changes, which can have detrimental effects on fish.
The most frequent times that fish keepers alter their pH without realizing it is when adding new fish from the store or when performing water changes.
GH and KH
The general hardness (GH) refers to the total amount of mineral salts present in the water. Water of GH 0dH to 4dH is very soft while water of GH 20 dH and up is very hard.
The carbonate hardness (KH) refers to the amount of carbonates and hydrogen carbonates present in the water. KH is important in aquaria because it can affect pH as well as the amount of CO2. KH has the same scale as GH.
Both GH and KH are important factors to having healthy fish for the same reasons as pH. Stable GH and KH are important as are maintaining levels similar to the levels the fish are used to in their native habitats. Once again, with more and more fish being bred domestically, the importance of maintaining native habitat GH and KH levels is becoming less and less important.
Dissolved Organic Compounds
Besides ammonia that fish excrete, many other toxins exist in the aquarium and gradually build up over time. These toxins can enter the aquarium through sources like fish food, chemical additives and even tap water or can be a by-product of the fish’s metabolism. Compounds like anti-metabolites, toxins, colorants, hormones can all have a profound effect on fish health, rates of growth and longevity.
These DOC’s can only be removed through water changes and can build up quickly in overcrowded tanks or tanks with big fish.
Temperature
The temperature in most aquariums is around 78 degrees Fahrenheit. Some fish like slightly cooler water, some slightly warmer but the most important factor for all fish is stability. A fluctuating water temperature can stress fish out leading to diseases and parasitic infections.