The Nitrogen Cycle

Updated: Jul 26

The nitrogen cycle is the process in which beneficial bacteria (Nitrosomonas and Nitrobacter) turn fish waste, uneaten food and decomposing matter (which produces ammonia) into less harmful waste into nitrites and then nitrates (the least harmful waste product). Cycling a fish tank refers to the process of establishing and maturing the biological filter, aka allowing beneficial bacteria to colonize your tank and your filter media. And no, running the filter on a tank overnight does not mean your tank is cycled. The time it takes to cycle your tank can vary from instantly or up the 6 weeks or more depending on which method you use to cycle your tank, which we will talk about later.

An Illustration of the Nitrogen Cycle

Stage 1: Ammonia source

The most common source of Ammonia in tanks is fish waste. Fish eat food, which turns into poop. Excess food which is uneaten or decaying matter will break down into either Ammonium or Ammonia depending on the pH of your water. If your pH is under 7, it will break down into Ammonium, and if your pH is over 7, it will break down into Ammonia. Although it is never good to have any Ammonia in an aquarium, if your water is acidic (less than 7pH), it’ll technically be less bad than if your water was more alkaline (over 7 pH). Either way, we never recommend there to be any Ammonia present for long in an aquarium. Ammonia is very toxic to fish, and even a slight presence of Ammonia (such as 0.25ppm) can burn a fish’s gills and skin, and eventually lead to death by Ammonia poisoning.

Stage 2: Ammonia being broken down into Nitrite by Nitrosomonas

The first group of beneficial bacteria, Nitrosomonas, will break down (or oxidize) Ammonia into Nitrite, which is slightly less harmful to fish, but still harmful, nevertheless.

Stage 3: Nitrite being broken down into Nitrate by Nitrobacter

The second group of beneficial bacteria, Nitrobacter, will then break down the Nitrite into Nitrate, which is not as harmful to fish as Ammonia and Nitrate but is still harmful in large amounts (generally 20-50ppm depending on the fish, as some fish or invertebrates may be more sensitive to Nitrates).

Stage 4: Getting Rid of excess Nitrate

Option 1: Partial Water Changes

There are two ways that Nitrate can be removed from aquariums. The first most common way is through partial water changes. Depending on the Nitrate levels in your tanks, you might need to do 20-30% water changes weekly, biweekly or even monthly. Most people usually recommend 20-30% water changes either weekly or biweekly, or even splitting it up into 2 small 10-15% water changes weekly. If your Nitrate levels are dangerously high, you might have to do even bigger water changes (30-90%) if your tank is overstocked or you tend to overfeed your fish (which you should never do). It is never recommended to do big water changes unless necessary, but if you have to, most people recommend splitting a big water change into two days (or more), such as 50% on the first day and 50% the day after. We recommend getting a gravel vacuum when doing water changes to help get any debris or uneaten food out from your substrate.

Option 2: Adding Live Aquatic Plants into your Tank

This leads us to the second method of reducing Nitrate, by having live aquatic plants in your aquarium. Aquatic plants use nitrates as fertilizer to grow, hence reducing the amount of nitrate in your tank. This is the easiest way of keeping your nitrate levels low and is also better for the fish as it simulates their natural environment. We personally recommend keeping fast-growing aquatic plants such as Anacharis or Hornwort, as they are very efficient in “sucking up” nitrate in your tank. Another aquatic plant we recommend are floating plants such as Duckweed, Salvinia Minima or Frogbit, which we would say are the most efficient in reducing Nitrate levels. Ultimately, having a planted tank means that you generally will have to do less frequent water changes, as water changes are mainly to reduce nitrate levels, which plants help to take care of.

The only downside to keeping plants like these is that you will have to trim plants like Anacharis or Hornwort often or having to remove excess floaters to prevent them from taking up too much surface area. Yes, plants require a bit more effort when having to maintain your aquarium, but we believe the benefits of having a planted tank far exceeds any downsides by a landslide. If you are still persistent in keeping plastic plants rather than live plants (no judgment!), we would highly advise against overstocking your tanks and doing more frequent water changes to ensure your Nitrate levels are kept below 20ppm.

If you would like to know more about cycling a tank, please read our blog post on Cycling an Aquarium!

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