consider when keeping your water and livestock healthy. One of the most common issues that come up for beginner and experienced aquarists alike is high nitrate levels. That is, a build up of fish and reef waste in the water column that can negatively impact many species of fish and corals, but in small quantities is required for everything to survive. In this blog we shall discuss the workings of the Nitrogen Cycle in a reef aquarium, what causes Nitrate to build up, the common consequences of high nitrate levels, and how to effectively control said levels.

The Nitrogen Cycle is broken down into three parts, Ammonification, Nitrification and Denitrification. In the beginning, fish and reef waste is excreted into the water column wherein it is very quickly broken down by many different species of bacteria into Ammonium (NH₄⁺) groups.

These ammonium groups are highly toxic,  but when an aquarium is cycled they are rapidly oxidised to Nitrite (NO₂−) groups by a species of bacteria called Nitrosomonas. This species of bacteria requires aerobic (oxygen rich) environments and phosphates in order to .grow and reproduce. As such, Nitrosomonas is typically found on exposed surfaces and in the water .column. The next stage is performed by a species of bacteria called Nitrobacter, another aerobic species, which further oxidises Nitrite to Nitrate (NO₃-). Nitrate is the least toxic form of waste present in the aquarium, and will hang around orders of magnitude longer than ammonia or nitrite.

The final stage of the cycle is Denitrification, this is where it gets interesting. There are many ways to remove nitrate from a system but the natural way it leaves is by a chemical reduction to Nitrogen Gas by various species of anaerobic bacteria, that is, bacteria that exist in the absence of oxygen. These are typically found in deep sand beds and porous live rock, where oxygen doesn't penetrate.

As simple as this cycle seems, it is a highly inefficient process. It takes time for a cycle to establish, and it can take some time for nitrate levels to reduce in the event of a disturbance.

Being a product of waste, nitrates can build up simply from overfeeding your aquarium. As well as uneaten food settling to the bottom, many fish typically never stop eating food once its offered, and can quickly pollute the tank with undigested food during excretion. Other sources of high nitrate can include disturbances such as fish and coral deaths, or inadequate water parameters resulting in a die off amongst tank inhabitants of all kinds.

Poor flow can also cause nitrates to spike locally in areas where the water isn't being moved and circulated, due to poor nutrient export. A common symptom of this is a bloom of cyanobacteria and/or dinoflagellates, which can quickly invade a host aquarium if not adequately controlled. This is something we will touch on in a future blog.

A third cause is inadequate filtration, which is something we've already discussed in a past blog. A filtration system is akin to the liver, kidneys and spleen in humans. If these organs don't work properly, waste builds up in our blood and makes us very ill. If you remember that the water is the lifeblood of any tank, the consequences of poor filtration are easy to imagine.

When nitrate levels are excessive, many problems can occur but the most common are the deaths of sensitive tank inhabitants such as corals and invertebrates, which cannot tolerate high levels for long. Disease can become more prevalent amongst fish, as high nitrates make it easier for contagions to grow and reproduce. Finally, high nitrate can lead to excessive algae blooms, which are one of the leading causes for an aquarist to quit the hobby, since these can often be a nightmare to control.

In essence, getting the balance right as early as possible will give you the best chance of success, prevention is worth a thousand cures.

The first, and most obvious way one can control nitrate levels is with regular water changes. Changing 10 to 20% of the aquarium's water volume per week is ideal for preventing a buildup of nitrate in the aquarium and comes with additional benefits such as the removal of toxins. For elevated nitrate levels, a large water change may be performed, but it is always best to try to identify the source of the high nitrate before resorting to such drastic measures. One of the easiest ways to go about this is to look at the tank as a whole and see what you can do to make life easier on the system.

When it comes to the filtration systems, in addition to having a good quality filter system in the first place, there are many things we as aquarists can do to make it easier for our tanks to process waste. The addition of live rock, deep sand beds and hyper porous stones such as bio blocks and bio spheres can be done with any type of tank and will greatly aid in the production of denitrifying bacteria.

For sumped aquariums, filter socks are a great way to improve filtration, by capturing large solids that fall in from the tank, giving bacteria plenty of time to process waste whilst keeping the water column crystal clear. Also for sumped aquariums, the addition of a good protein skimmer is a must. This machine makes use of a pump driven venturi to agitate water as it is sucked in to the skimmer chamber, resulting in floating solids settling as a fine foam at the top, removing a lot of waste and nitrogen fixing bacteria in the process.

One way to speed up this process of skimming out waste is by the use of chemical nitrate removers such as Red Sea NO3PO4X . These chemicals provide a source of carbon, which encourages the rapid growth of denitrifying bacteria and various other species of nitrogen fixing bacteria, hence why this practice is often referred to as carbon dosing. These rapidly build up in the water column and are readily removed by protein skimming, hence a good protein skimmer is required for any of these chemicals to work.

Finally, a novel but effective way to reduce nitrates in a sumped aquarium is by growing macroalgae species under a good quality sump light. These are typically species of Chaetomorpha and Caulerpa, which take up nitrates from the water column as they grow. In ideal conditions, these macroalgae can grow rapidly, and are a great way to prevent outbreaks of cyanobacteria and dinoflagellates as they essentially outcompete those species for nutrients. Be sure to periodically harvest excess macroalgae in order to prevent overgrowth and to physically remove nitrates from the system.