The Nitrogen Cycle

The nitrogen cycle is probably the single most important biological process in the aquarium. Whatever food is introduced into the tank will eventually undergo biological decay via a process called the nitrogen cycle.

Waste food, faeces, and dead organisms rapidly undergo a biological breakdown producing ammonia which is extremely toxic to marine life. Luckily it is eagerly eaten by bacteria producing Nitrite which whilst still toxic is markedly less so. Further bacteria then turn the nitrite to nitrate which is a far less toxic and most aquariums contain fairly high levels of nitrate.

The cycle of food - ammonia - nitrite - nitrate is absolutely fundamental to maintaining a healthy aquarium and commonly takes 3 to 4 weeks to become established as colonies of bacteria grow to meet the demand.

A number of filters have long been used to facilitate this cycle. The conditions required for bacterial growth are food, oxygen and water movement. Undergravel filters were the 1st breakthrough. A plate was fitted beneath the substrate, through 100s of tiny holes water was pulled downwards through the substrate to the plate where vertical outlets came up to powerhead pumps. This created a rapid flow of oxygen rich water through the substrate which allowed for a massive surface area to be colonised by areobic nitrogen reducing bacteria. However, the substrate rapidly became clogged with detritus and within a year or so would become a huge nitrate bank, leaching nitrate into the water.

The next major advance was the wet/dry filter. Usually a vertical column outside of the tank filled with a suitable material that provided the largest possible surface area with a trickle bar above allowing just enough water flow to keep the substrate wet without total immersion. these proved enormously efficient and allowed a much greater range of creature to be kept. Nitrate levels would, however, still rise. Large and regular water changes were required to export nitrate.

Various other methods have been employed to check nitrate levels to allow the keeping of more sensitive creatures, especially hard corals which cannot tolerate nitrate levels above 5ppm. Algal scrubbers, a corrugated tray with a flow of water beneath a permanent light allowed the growth of hair algae which could then be harvested. All plants use nitrate and algal scrubbers are effective at lowering nitrate levels but also take up large quantities of minerals which then have to be replaced with supplements.

More recently advances have been made in allowing anaerobic conditions to exist which creates the necessary conditions for another bacteria to eat nitrate producing nitrogen gas which then harmlessly bubbles through the substrate to the surface.

In tropical tanks this was acheived by using large quantities of 'live rock'. This is rock formed by dead coral. Extremely porous it allows a slow transfer of water through its centre and has a vast surface area relative to its size. Aerobic bacteria live within the 1st few milimetres and brakdown soluble waste to nitrate which deeper in the rock where no oxygen is present is further broken down to harmless nitrogen. A similar process can be created using a deep live sand bed.

I try to create these conditions in my native marine tank by using live sand taken at low tide with plenty of the underlying black sandy mud, rich in micro-organisms. Also porous rocks such as chalk from below the tideline will already have colonies of all the necessary bacteria to carry out the nitrogen cycle.

Seeding a new aquarium with live materials such as rock, sand and seawater eliminates the wait for colonies to develope before introducing livestock.

To effectively encourage nitrate eating anareobic bacteria to thrive a deep sand bed is required. Remove any filters that are designed to creatre nitrate so that the process can be carried out in the sand and rocks. Encourage the growth of macroalgae. Use skimmers to remove as much soluble waste as possible initially.

Using this system I have been able to keep nitrate levels below 5ppm for over a year with no water changes.

The next step is to reach the stage where the skimmer can be turned off which will allow the growth of plankton allowing the keeping of bivalves, soft and hard corals and sea pens etc. I intend to expand my system as follows:

Build a much larger display tank - 6' x 2'6" wide and 3' deep with halides above. The tank will be drilled allowing a 2" pipe to fall to my current 3' x 2' x 2' tank below , which will become my sump tank. The sump will have an 8" live sand bed covered in porous rock such as tufa with permanent lights over for plant growth. A return pump will then go back to the new display tank.
The display tank will have a 10" sand bed and landscaped in live rock. All water will be taken directly from the sea. Circulation will be carried out by 4 no. 5800 l/h pumps. only 1000 l/h will pass through the sump allowing plankton to develope which will trickle up to the display tank feeding the livestock.
It is hoped that skimming will not be necessary. All of the nitrogen cycle will take place slowly and naturally with excess nitrate being taken up by the strong algal growth in the sump which can be harvested and disposed of as and when necessary.

I had a similar system in my tropical marine reef tank for many years, mangroves were grown in one of my 3 sumps which are excellent nitrate exporters. No British equivalent to the mangove exists - but perhaps an eelgrass bed could be maintained?