In the marine aquarium, be it a reef tank or a fish only aquarium, the nitrogen cycle is of paramount importance. It is the life support function for all the livestock. Without it, or with a serious deficiency, the livestock will be in serious difficulty and face death.
In an aquarium the livestock carry on with their normal life functions and need feeding. This introduces waste into the seawater. In addition, algae etc can die and rot. The product of the breakdown is ammonium (NH4+). Because seawater is at a fairly high pH (around 8.3) some of this ammonium is present in the form of ammonia (NH3). That is the problem, ammonia.
Ammonia that is present even in small amounts is toxic – deadly – to aquatic life. Fish can display ammonia poisoning by peculiar swimming: Swimming slowly and rolling over, going up and down in a random fashion, breathing very quickly, lying at the bottom, floating around at the top etc. Varied, but if seen there can be no doubt something is wrong. If only one fish is behaving strangely, and the rest are normal, the problem is unlikely to be ammonia poisoning.
Nature is ready to deal with the problem, luckily for us aquarists. There always seems to be some kind of life that is ready to use up whatever is available, and with ammonia this is the case. The life forms are Nitrosomonas bacteria. The ammonia is used by the bacteria for energy.
So that’s fine then, problem solved. Not quite. Even though the bacteria mentioned have removed ammonia, it hasn’t been conjured into oblivion. Guess what, the ammonia has been converted to nitrite (NO2) which is nearly as toxic as ammonia. Fish reaction to nitrite is similar to ammonia.
So what we have is a cycle, as in the heading. More bacteria use the nitrite, these bacteria being Nitrobacter. The situation becomes much better after this stage, as the product produced is nitrate (NO3). This is nowhere near as toxic as ammonia and nitrite, but can cause some problems at high levels. Probably the major problem that aquarists could face with nitrate at lesser levels is excessive algae presence.
Nature however can once again come to the aquarist’s aid because, yes that’s right, bacteria can deal with the nitrate. Hang on a moment though, there’s a qualification here. It depends on the bio-filtration being used (the bio-filtration is home to the different bacteria). If the aquarist is using live rock, fine, good quality live rock in proper quantity will deal within reason with nitrate. If the bio-filter is a canister, for example, then there aren’t any bacteria present to deal with the nitrate. Why? Because the environment is oxygen rich. To deal with nitrate a nil to low oxygen environment is required. Live rock can provide this as the bacteria involved live deep inside. If the example a canister filter is in use then other means are available if required to control the nitrate.
The nitrate is removed because the bacteria extract oxygen from it, this being the only way open to them. The end gas, nitrogen, is released from the seawater at air/water interfaces.
So the whole nitrogen cycle is from ammonia to nitrogen gas. There are several ways to achieve the cycle, but whatever method is chosen (except perhaps one) there are important points for the aquarist to be aware of.
The bacteria colonies that convert ammonia to nitrite use a lot of oxygen in the process. The next stage, nitrite to nitrate, uses up a lesser amount. There is only one place that the oxygen can come from and that is the seawater. In addition to the bacteria, the reef livestock need oxygen in good quantity. So the seawater must always have a good oxygen content. Having said that, the modern reef or fish only system usually hasn’t a problem, as they are mainly open topped, often use weirs, and have good water circulation, particularly reefs. If oxygen content were ‘near the edge,’ it could be more of a problem for a fish only system because of the usually higher fish stocks per gallon. What is the possible exception mentioned in the previous paragraph? The trickle filter. Seawater is exposed to the air as it runs down through the media, which means oxygen from the air could be used (an air/water interface).
The nitrogen cycle bacteria are not present instantly. With ‘unnatural’ filters, such as the example a canister, they are introduced by inoculating the seawater with the required substances. Commercial preparations are available. Measuring the seawater for ammonia, nitrite and nitrate advises the aquarist when the bacteria colonies are working. Live rock is different, as the required colonies of bacteria should be present. Caution still has to be exercised as these colonies might have been diminished in the ocean to aquarium processes.
There isn’t any argument – the life support system, or bio-filter, or nitrogen cycle, call it whatever, must be present and effective. It will then protect the livestock by working unseen and silently.