It is essential to have efficient biological filtration in any marine aquarium. This is the bacteria based ‘filter’ that converts dangerous toxins so that the livestock are not threatened. Some aquarists prefer to call this bacterial function ‘life support’ and this name seems appropriate. Without it, or with insufficient, the livestock would die or at least be sickly.
Biological filtration occurs in an area where bacteria can live and perform their functions. The most essential bacteria need a plentiful supply of oxygen and a suitable media. The bacteria will not only live in and on the media provided by the aquarist, but on many surfaces in the aquarium. The media is the main area though.
There are different ways of housing the bacteria. One of the best ways, probably the best, is to use live rock. This is rock that has either been harvested from the wild or aqua-cultured and already contains a population of bacteria. Another way is to use a container, for example a canister filter, to provide a home. This kind of filter requires suitable media.
So what is this ‘cycle’ of the aquarium? It refers to the Nitrogen Cycle, which is the overall title given to the actions of the bacteria that are so important. Basically, ammonia is produced by livestock and is lethal. Bacteria convert the ammonia to nitrite, another toxin that is nearly as lethal. More bacteria convert this nitrite to the generally non-toxic nitrate.
Live rock as said already contains bacteria, so perhaps there isn’t any need to ‘cycle’ it before use. This in theory is correct; however care needs to be taken. Live rock will normally be purchased from a dealer, and it will have been ‘cured’ (that is, given time for all the dying life on and in the rock to clear). Once the curing is complete the rock is ready and has a good bacterial load. However, as with all life, bacteria need food. For the most part, cured live rock is placed in aquariums or vats awaiting sale and is not fed. Over a period of time, the bacterial population reduces. It is important therefore not to assume that just because live rock is in the aquarium all is well and fish can be added. The bio-load, particularly fish, must be added slowly over a lengthy period of time so that the bacterial population has time to adjust. Failure to do so is likely to result in major problems or disaster, as the bacteria that face a bio-load that is too large cannot develop their numbers quickly enough, and therefore cannot deal with the amount of toxic ammonia that is present.
If the aquarist is to use a device such as a canister filter then, as said, a suitable media is required – this type of media is readily available. In this case, the media will not contain any bacterial life whatsoever and it is up to the aquarist to kick-start it. This is easily done.
It used to be achieved, and this method is still sometimes recommended, by placing one or two hardy fish in the aquarium. These fish can hopefully withstand the rise in ammonia until the bacteria become established and deal with it. However, this method is not recommended. It is unethical to subject any life form to unnatural and stressful conditions.
Instead, the method recommended is to purchase a bottle of commercial maturation fluid. These are readily available and not expensive. The instructions should be followed carefully. Basically, the aquarist is introducing ammonia into the aquarium which of course is food for the first bacteria that will deal with it. It takes time for the bacteria to develop and a test kit is used to monitor the ammonia level – when it reaches a certain point additions of the maturation fluid are stopped. There will eventually be a drop in the ammonia level and another test kit will detect nitrite. This means that the cycle is beginning – bacteria are converting the ammonia. Then nitrite will start to be converted. Monitoring continues until tests read zero. Sometimes levels fall slowly but often they suddenly drop. This is because the bacteria population is large enough to deal with all the toxins present. At this point the bio-filter is said to be mature, meaning it is capable of supporting life. However, it is still unstable and stocking of the aquarium needs to be carried out slowly to allow the bacteria to adjust.
One point about maturing a bio-filter using maturation fluid. As said, bacteria convert ammonia to nitrite, then nitrite to nitrate. Nitrate, though not generally toxic, is undesirable and reduces the quality of the seawater. Clearly if bacteria are converting ammonia and nitrite then nitrate is going to appear. Nitrate is easily tested for and if present in excess a seawater change will dilute the concentration. There is another way to avoid nitrate in the aquarium entirely during the maturation period and that is to use a bucket. Put seawater in the bucket and a heater. Run the seawater to the canister or similar filter and then back again. The filter can be matured this way and when done the seawater in the bucket can be thrown away. The bacteria populations are in the canister and when connected to the aquarium are ready to work.