When a new tank / system is set up, it will be referred to that it goes through a cycle. What that refers to is a process / period required for the population of bacteria to reach the appropriate size to fully establish the nitrogen cycle.
The during the cycling time, ammonia levels will increase then fall, followed by nitrite rising and falling, then finally nitrate will appear and start to increase. Once all traces of ammonia and nitrite are gone and nitrate is detected, then the initial cycle is said to be over. See the graph to the right that indicates how the concentrations increase and decrease during this initial cycling period.
A broader application of the term cycle is the process that any organism population undergoes to reach an equilibrium state. See Ecosystem Population Dynamics for some more general comments on this.
There are a lot of different ways in which a tank can be cycled, including what is used as the nutrient source, which pieces of equipment are utilised, water changes etc. No piece of equipment or methodology is compulsory when cycling, the liverock will go through the stages of organisms dying, producing waste, and then the bacteria building up to process it, regardless of what is done.
However, the goal is typically to retain as much life as possible that comes on the liverock, otherwise its pointless buying live rock to begin with. To achieve that, stress needs to be minimised on any livestock within the system (which includes all organisms, including the huge number of hitchhikers that are generally not observed) by keeping nutrient levels (including ammonia and nitrite) as low as possible. The best method involves significant water changes, aggressive protein skimming and another other nutrient exporting mechanism available. This will increasing the survival rate and subsequently increase the system stability in the future.
One of the most common methods is to simple add the newly purchased liverock to the system. In most cases some of the organisms on the rock will die off, due to stress during transport or their lack of suitability for the captive environment. This provides the source of ammonia to get the bacteria multiplying.
This involves simply adding a prawn to the system, allowing it to decay, thereby feeding the bacteria in the cycle. A more efficient method is to feed the tank each day a small amount of food as if there were a small number of fish in the system. That will spread the nutrient spike out some more.
The recommended method currently is to use a combination of the dead prawn and liverock techniques. This is preferable as when the liverock is added there is already a substantial biological filter present, significantly (if not totally eliminating) reducing the severity of the cycle. The steps involved are:
- sand / substrate are added and tank filled.
- feed the tank for a couple of weeks to establish a bacteria population.
- allow the cycle to complete, indicated when nitrite falls to zero.
- add some liverock.
- allow the system to cycle again, if it does.
- then add some more liverock if required.
Do perform these during the cycling process, especially if using liverock. Removing the nutrients from the system and keeping their levels down will assist in more life on the rock surviving the process. It will not increase the time it takes for the cycle to happen. Perform the largest percentage (25%+) water changes that can be done, as this has the most impact on the concentration of nutrients. Smaller percentage water changes have little effect.
It is also highly recommend to have the protein skimmer, if one is being used on the system, operational from the start. Removing those organic compounds from the system mean they are not processed by bacteria, lower the nutrient levels.
It is also highly recommend to have the lighting on during the cycling process to assist in preserving as much life as possible. The liverock will be covered with photosynthetic organisms (including corals, macro algae and coralline algae) and having the lights off will starve them of the light they require to live. The cycling process can be stressful enough for organisms, without adding a lack of lighting to the mix as well.
What To Expect
Cycling a tank can take anywhere from a couple of days to a couple of months, depending on the methods used and other factors such as cured or uncured liverock. The typical length is 2-3 weeks and during this period a lot of changes happen. Factors such as lighting (intensity and length of photoperiod), filtration and water changes can effect what is observed during the initial cycle period. The following is a guide only.
Liverock is one of the more common methods of cycling a tank. One of the first things that will happen if the liverock is not cured or transports poorly, is some of the organisms on and within the rock will die off. This can be due to two things, stress from transportation and storage or not being suitable for captivity. The rock will be covered with and full of organisms such as algae, bacteria, invertebrates such as snails, crabs, starfish, brittle stars, bristle worms, and corals. It is not uncommon to see these organisms after the rock is added.
The death of some of the organisms and metabolism of those still living produce waste, with the most important one being ammonia. Straight away or within a couple of days an ammonia test kit should be giving a reading. If an ammonia reading is not detected within that period, then it may be that part will take longer or had already occurred before the first reading was taken. As soon as ammonia is present in the system, bacteria that utilises ammonia will start to multiply and produce nitrite as a product.
The next step is an increase in the nitrite level as the ammonia metabolising bacteria build up their population. While the nitrite level is increasing, the ammonia level should start to fall, eventually to zero. Again, this may take a couple of days or weeks. The nitrite metabolising bacteria then start to multiply, using up the nitrite and producing nitrate as a by-product.
What is typically termed as the “final” step, however some other cycles do continue for some time, is the increase in nitrate levels and subsequent decrease in the nitrite to zero. This can take from days to weeks to occur.
It is only when the ammonia and nitrite readings have reach 0 ppm that the initial cycling process is over. This is the point at which many jump in and add fish. However, it is prudent to wait for longer and allow the system to settle down further before making any livestock additions.
During the cycle, especially after the ammonia spike, algae will start to grow. And in some cases, very prolifically. At this point in time, the system has high nutrient levels, and the algae takes advantage of this. This is a reason why it is good to perform water changes and ensure have control of nutrients entering the system (via water changes and evaporation top off, such as using RO over tap water).
The first algae seen is a brown algae that covers the rock, sand and glass. After a couple of days to weeks, this will start to be replaced by another type of algae, commonly green. But may also get red algae blooms appearing. With time and good maintenance regimes, these blooms will settle down and disappear almost entirely. When things have settled down, the more desirable algae, coralline algae, will start to grow and dominate. This appears as a pink / purple coating on the liverock. During the algae blooms and the cycling process, a bloom of bacteria or phytoplankton may occur. This will turn the water cloudy for a couple of days, either white or green. Nothing to worry about, unless it lasts for a significant length of time or occurs to the system well after the initial cycling period.
Every cycle is different, so there is no set time table or strict order to follow. It may be found that your system differs slightly or even significantly to the above. This is totally normal.