The Nitrogen cycle
Waste produced by fishes is immediately broken down into highly toxic ammonia. The ammonia produced is the food source for a type of bacteria that converts ammonia to toxic nitrite. Another type of bacteria would convert this nitrite to less toxic nitrate. These groups of bacteria are collectively known as nitrifying bacteria and they are essential for life to be sustained in an aquarium. Without them, toxic ammonia and nitrite would build up to lethal levels and poison your fishes, causing them to die. Most experienced aquarists would agree that ammonia and nitrite levels in an established tank should be undetectable by aquarium test kits. Make sure your tank has undetectable levels of ammonia and nitrite before adding your beloved fishes in!
What about Nitrates?
Thankfully, nitrates are way less toxic than ammonia and nitrite. Although there are bacteria that break down nitrates in the aquarium, they can only be found in certain biological media and deep substrates where anaerobic conditions are created. Most aquarists depend on good water changing schedules and aquatic plants to remove nitrates from the water. So, please remember to do your water changes!
“Cycling” the tank
If you have done some readings online or watched some videos about fish keeping, you may have come across the phrase “Cycling the tank”. This is an important step of starting up any aquarium as it allows the nitrifying bacteria to inhabit the filter. Conventionally, aquarium owners would set up the tank without adding fishes in for a few weeks to allow the nitrifying bacteria to establish in the filter. However, many new aquarists do not have the patience and give in to temptations by adding fishes into the tank early. When the fishes are added too early, the nitrifying bacteria population would not be enough to support the waste the fish produced, leading to an increase in ammonia or nitrite levels which can kill your fishes. Thankfully, gone are the days of waiting weeks for the tank to cycle as there are now over-the-counter nitrifying bacteria available which would allow you to cycle your tank immediately. This will enable you to add your fish to the tank right after setting it up!
A large portion of the nitrifying bacteria in the tank inhabits surfaces on the biological media in your filter as water is constantly flowing over them, bringing oxygen to the bacteria to allow them to detoxify ammonia and nitrite. Examples of biological media are ceramic rings, crushed lava rocks, and course sponges. You may often realise that some biological media are way more expensive than others in the aquarium store. They differ by the total surface area available for these nitrifying bacteria to inhabit. A better media would have a greater total surface area, therefore having better biological media (especially in a small filter) would allow you to fully utilise the space in your filter. This makes the filter more efficient at carrying out the nitrogen cycle, detoxifying the waste produced by your fishes. That’s great news, as you can now add more fishes into your tank!