Nitrogen Cycle

Nitrogen cycle in reef aquaria refers to the process by which biological waste by-products are converted from toxic substances to harmless compounds through the actions of bacteria. In a properly setup tank, this process occurs naturally through the actions of bacteria that colonize different areas of the tank. Establishing this process in the first place is known as ‘cycling’ the tank.

Some call it the biological cycle, the nitrification process, new tank syndrome or even the start-up cycle. They all are referring to the same cycle – The Nitrogen Cycle. The aquarium nitrogen cycle is a very important process for the establishment of beneficial bacteria in the aquarium and in the filter media that will help in the conversion of ammonia to nitrite and then the conversion of nitrite to nitrates.

This process can take from 4 weeks to 8 weeks or longer to complete. It is vital for anyone planning on keeping aquarium fish to understand this process. Learning about this process will help you to be successful in keeping fish and it should definitely improve your chances when keeping marine fish. The best way to monitor the nitrogen cycle is to purchase an aquarium test kit that will test for ammonia, nitrites, nitrates and ph.

Test your aquarium water every other day and write down your readings. You will first see ammonia levels rising. A few weeks or so later you should see the nitrite levels rising and the ammonia levels dropping. Finally, after a few more weeks you should see the nitrate levels rising and the nitrite levels dropping. When you no longer detect ammonia or nitrites but you can detect nitrates you can assume that it is safe to add your fish.

First, an understanding of the steps involved in the nitrogen cycle is required in order for the hobbyist to successfully monitor the cycle and know when the tank has successfully completed its cycle.

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 which 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 which 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.

Establishing the nitrogen cycle in the reef tank

It should be somewhat obvious that to establish the bacteria which convert ammonia to nitrites, a source of ammonia must be added to the tank. Once these bacteria start producing nitrites, the bacteria which convert nitrites into nitrates can start to establish themselves and of course, once nitrates are available, the nitrate converting bacteria can start to establish themselves, providing the hobbyist has taken steps to provide a suitable oxygen poor home for them.

The ‘trick’ to establishing the nitrogen cycle in the tank is to do it without endangering any tank inhabitants. This generally means that the part of the cycle which converts ammonia to nitrate should be established before any specimens are added. Fortunately a ready supply of ammonia is introduced with the live rock that is introduced into the system. As the live rock goes through its curing process, the decaying life forms on the rock provide the starter fuel for ammonia and nitrite consuming bacteria to colonize the rock. When live rock is being cured and this process is getting setup, toxic levels of ammonia and nitrite can form and specimens cannot be introduced until it is verified that both ammonia and nitrite have dropped to zero levels through the use of test kits. Typically, while this process in occurring, the hobbyist will measure an increase in ammonia and then it will start to drop as ammonia consuming bacteria start to grow. The hobbyist will then measure an increase in nitrites as the ammonia gets converted to nitrite. As the nitrite consuming bacteria start to grow, the nitrite level will also start to fall. When both ammonia and nitrite levels fall to zero levels, the cycling is complete. Typically, the nitrates will be high at this stage and the water that was involved in establishing the cycle should be replaced with new saltwater.

Once the live rock is cured, the basic tank cycle has been established and the live rock can be stacked in its final arrangement in the tank. Even though the basic bacteria types have been established, the number of bacteria will fluctuate depending on the bioload of the system. Also, there is probably not much bacterial colonization of the sand bed at this point. Therefore it is important to increase the bioload of the system slowly so that the bacteria colonies can grow to match the load of the system. If a lot of fish are added to a newly cycled tank, the sudden increase in waste products will cause a new mini cycle to start all over and since there are specimens in the tank, they are at risk of death or injury due to the ammonia or nitrite spikes that will occur. Corals and clams do not generally add bioload to the system, so they can be added more freely than fish or other critters that require constant feeding.

The final part of the nitrogen cycle (converting nitrates to nitrogen gas) has to be established after the tank is setup. The first thing that a hobbyist must do is to ensure that the reef tank provides oxygen poor regions in the live rock and sand. Old school was that this was to be avoided at all costs due to the concern over noxious gases, such as hydrogen sulfide, being formed. This concern seems to be overly exaggerated and can probably be ignored for the most part. It is important however, that once these oxygen poor zones are created, that they not be unduly disturbed.