The single most important concept for any aquarium. Understand what it is, why it matters, and how to cycle your tank before adding any livestock.
The nitrogen cycle is the biological process that converts toxic fish waste into relatively harmless compounds. Fish excrete ammonia (NH₃) directly into the water, and leftover food also breaks down into ammonia. Without a functioning nitrogen cycle, ammonia accumulates and will kill your fish within days.
The cycle relies on two groups of beneficial bacteria that colonise your filter media, substrate, and live rock. These bacteria don't come pre-packaged — you have to give them time to grow. This is called "cycling" your tank.
In a new tank with no beneficial bacteria, ammonia rises rapidly from fish waste, uneaten food, and decaying organic matter. You can seed ammonia intentionally by adding a small pinch of fish food daily, or a few drops of pure ammonia.
You want ammonia to reach around 2 ppm before the bacteria start catching up. Test daily.
The first bacterial colony, Nitrosomonas (and related species), begins converting ammonia into nitrite (NO₂⁻). You'll see ammonia levels drop while nitrite spikes. Nitrite is also toxic — it interferes with how fish blood carries oxygen, causing a condition called brown blood disease.
The second bacterial colony, Nitrospira, converts nitrite into nitrate (NO₃⁻). Nitrate is far less toxic than ammonia or nitrite and can be managed with regular water changes. When you test 0 ammonia and 0 nitrite after a dose of ammonia, your tank is cycled.
Nitrate itself should be kept below 20 ppm in a reef tank, and below 40 ppm for fish-only systems. Freshwater planted tanks benefit from some nitrate (5–20 ppm) as plant fertiliser.
Several methods can cut cycling time significantly:
— Seeded filter media: Get a handful of established filter media (sponge, bio-balls) from a healthy tank. This transfers live bacteria instantly.
— Live rock: In reef tanks, cured live rock carries a massive bacterial population. A tank set up with live rock can cycle in as little as 2 weeks.
— Bottled bacteria: Products like Dr. Tim's Aquatics One & Only, Brightwell Aquatics MicroBacter7, or Seachem Stability contain live nitrifying bacteria and genuinely work. Follow dosage instructions carefully.
— Higher temperature: Bacteria multiply faster at 27–30°C. Running slightly warm during the cycle (then dropping to display temperature afterwards) can shave off a week.
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