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Freshwater Cycle
Quite often, we've heard people refer to the Cycle of their aquarium. Nearly as often, we see newcomers to the hobby, and sometimes even seasoned veterans of the hobby ask, What is this "Cycle" of which you speak? Well, that is what we are here to learn.

The "Cycle" basically refers to the establishment of the Nitrogen Cycle or Nitrification Process that takes place within the aquarium. In the simplest of terms, this is the process in which toxins within the tank, which are generated from a variety of sources, are consumed and converted to far less toxic materials resulting in a suitable environment in which the inhabitants of the aquarium can survive.

This process begins with the introduction of your first fish, flake of food, or in some cases, your first plant. As fish waste (urine and feces) is released into your tank, a toxic chemical called Ammonia/Ammonium (Nh3/Nh4) is released into the water. Whether the material is converted to ammonium or Ammonia is dependent upon the ph level of the water. If the ph is under 7, you will have ammonium. If the ph is above 7, you will have ammonia. Unconsumed food, decaying fish corpses, and decaying plant matter are also sources that generate ammonia in the water. Though some fish are more tolerant of low levels of ammonia (these fish are often referred to as starter fish), it is still very poisonous to fish. This is where the first of two, very beneficial, bacteria come into play. As the ammonia becomes present in the tank, this bacterium, called Nitrosomonas, begins to develop and create colonies within the filter media, substrate, and any other porous materials within the water. The Nitrosomonas bacterium feeds on the ammonia, converting it to yet another highly poisonous toxin called Nitrite (No2). Many people choose a method called Fishless Cycling in which ammonia is added to an uninhabited tank in effort to eliminate the undue stress the fish are forced to endure during the Cycle, but that is subject for a different article.

The next step of this process is reliant upon a second beneficial bacterium. It was previously thought that Nitrobacter was the bacteria responsible for this step, recent research by Dr. Timothy Hovanec, Chief Science Officer of Marineland Labs, has discovered that the bacterium Nitrospira is the correct bacteria. As the nitrite is released into the water, the Nitrospira bacteria begin to develop and colonize within the tank, much like the Nitrosomonas, but at a much slower rate. These bacteria feed on the nitrite within the tank, converting it to Nitrates (No3), which although still harmful at high concentrations, are far less toxic than the aforementioned chemicals.

Sadly, there is no miracle bacterium to convert away the Nitrates within the tank. For that we must rely on live plants, which will convert some of the nitrates to plant matter, and the performance of routine water changes. Nitrates, although far less harmful to the fish, are still toxic and must be kept in check. Nitrates should ideally be kept at around 10-20ppm (parts per million). In addition to stunted fish growth, disease and undue stress, high levels of Nitrates can also lead to unsightly green water known as an Algae Bloom. Water changes should be performed at a rate of 15-20% every 2 weeks. This may vary based on the type, size, and number of fish you plan to keep with respect to the size of your tank.

Marine Reef Cycle Courtesy of usmc121581

Saltwater goes one step further, This is only for reef systems with a minimum sand bed depth of at least 3".

The nitrogen cycle is composed of 4 basic steps.

Ammonia: Ammonia (NH3) initially enters the system via biological waste. These wastes may come from fish poop or dead organisms, which are decaying in the tank. These wastes mineralize into the compound ammonia. Ammonia is a very toxic compound. Bacteria that colonize rock and sand surfaces utilize ammonia as food and convert it into a new compound called nitrite. Ammonia should always measure zero in a fully cycled tank.

Nitrite: Nitrite (NO2) is only slightly less toxic than ammonia. Fortunately, there are other types of bacteria that also colonize sand and rock surfaces, which consume nitrites as food and convert them into nitrates. Nitrites should always measure zero in a fully cycled tank.

Nitrate: Nitrate (NO3) is a relatively harmless compound. Nitrates tend to accumulate in the reef system if it is not setup correctly. Fish can tolerate fairly high nitrate levels, but most corals do not. Nitrate is also a plant fertilizer, so its accumulation can lead to algae problems. The brute force way to control nitrates is to do large water changes and therefore dilute the nitrate levels, but there is a better, more natural way to deal with nitrates. Nitrates in a fully cycled tank should ideally remain at zero, but up to about 20ppm is acceptable. Higher levels may lead to issues with coral health or algae growth in the tank. Some corals may actually benefit from the higher nitrate levels, but they are atypical.

Nitrogen: In a properly setup reef tank, the nitrates can be further processed by special types of bacteria which convert the nitrates into harmless nitrogen gases which escape into the atmosphere. When the process includes this step, the nitrogen cycle is completed and the tank will maintain zero nitrates without significant water changes or the requirement for specialized external equipment to remove it from the system. The key to this final step is to provide oxygen poor areas of sand or rock. The bacteria that perform this last step of the process only live in oxygen poor (anaerobic) areas of the tank. The surest way to establish these anaerobic areas is to include a sand bed that has sufficient depth and sufficiently small particle size to restrict water flow in the lower areas of the bed.

That’s it, Science lesson over. Happy Fishkeeping.

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