The Nitrogen Cycle

Possibly one of the most critical yet misunderstood components in managing a fish tank is the nitrogen cycle. The nitrogen cycle is responsible for your harmonious ecosystem. When the nitrogen cycle isn’t established or is interrupted the result is usually the death of your aquatic creatures.

Unfortunately, because most people aren’t aware of this natural cycle they fall victim to what is known in the fish tank community as “New tank syndrome”, which essentially translates to the poisoning of your fish until they can no longer sustain life.

So how exactly does the nitrogen cycle work? There are 4 main phases and we’ll break them down. Here are some terms and their identifiers that you’ll want to familiarize yourself with:

Ammonia (NH3)

Unionized ammonia is extremely toxic to fish even at low levels if the pH is alkaline(above pH: 7). It is excreted by your fish in their waste and generated by decomposing organic matter within your fish tank.

Ammonium (NH4)

NH4 is ionized ammonia and less harmful compared to NH3. It’s present in acidic water levels (below pH: 7). I’ve never maintained an acidic tank so this is merely informational.

Nitrite (NO2)

Nitrites are the by-product of NH3 oxidation performed by the Nitrosomonas bacteria. It is still harmful to your fish, however much safer than NH3.

Nitrate (NO3)

Nitrates are the by-product of NO2 oxidation performed by the Nitrobacter bacteria. This is the least harmful to your fish however it needs to be removed by water changes.

Now that you have a grasp on the terms and what each is/does let’s go over the cycle.

Phase 1:

Ammonia is created by fish waste and decomposing organic matter. That ammonia is extremely toxic and can kill your fish rapidly. Signs of ammonia poisoning are: gill damage and fish “gasping” for air at the surface of the water. Ammonia is battled by a bacteria called Nitrosomonas, nature’s way of creating a balance.

Phase 2:

The Nitrosomonas bacteria we’ve spoken about requires two things two survive: Ammonia and Oxygen. Nitrosomonas has a very important job which is converting ammonia (NH3) to Nitrites (NO2). Nitrites are much safer than ammonia (NH3) however they can be just as dangerous if uncontrolled. Unlike ammonia which causes physical damage to the fish thereby essentially suffocating them, nitrites cause irreparable damage to the internal organs of your fish. Nitrites are battled by a bacteria called Nitrobacter.

Phase 3:

The Nitrobacter bacteria operates similarly to the Nitrosomonas except it feeds off nitrites instead of ammonia. It creates nitrates (NO3) which are even less harmful then nitrites (NO2) for your fish. While nitrates (NO3) are the safest for your fish, they still require removal.

Phase 4:

The only things that remove nitrates (NO3) are plants and water changes. Unfortunately, most of us cannot maintain enough plant life to consume all nitrates so water changes are required.

Those are the 4 phases that complete the cycle. If we were to draw it out it would look something like:

Fish/Matter -> Ammonia -> Nitrosomonas -> Nitrites -> Nitrobacter -> Nitrates -> Intervention (water change).

You should have already established that each phase feeds off the previous, and because of this, it takes time to establish all the bacteria colonies to complete the cycle. This time is the difference between a cycled tank and an uncycled tank. Because of the quasi food chain outlined if you monitor the presence of NH3/NO2/NO3 during the cycling period and chart it you will see a chart that had increasing nitrites when the ammonia was decreasing and increasing nitrates when the nitrites were decreasing.

Eventually, there are enough Nitrosomonas to convert all ammonia, enough Nitrobacter to convert all the nitrite, and that dip of nitrates at the end is you doing your part via water changes. As you can see the Nitrates will forever be on an uptrend. Once your tank is cycled you should always have readings of 0 for ammonia and nitrites.

There are two ways to start the cycle: 1) add fish and allow their waste to generate ammonia (stocked cycle) 2) Add pure household ammonia to your tank daily until the cycle is complete (fishless cycle). I’ll write another article on cycling options but for now, just keep in mind the damage that can be done to your fish if you are not super diligent with water changes. Anytime you see Ammonia/Nitrite levels above 0.3mg/L you need to perform a water change.

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