The nitrogen cycle in aquariums

What the nitrogen cycle actually does

Every fish tank is a closed system. Fish excrete ammonia (NH₃) directly through their gills, and additional ammonia comes from uneaten food and decaying plant matter breaking down on the bottom. In open water, dilution handles that. In your 60-litre tank, it concentrates fast.

Ammonia is acutely toxic. At a pH above 7.5, even 0.25 ppm can begin damaging fish gill tissue. The nitrogen cycle is the biological process that removes it - not by destroying it, but by converting it through two bacterial steps into progressively less harmful compounds.

Stage one: Nitrosomonas bacteria oxidise ammonia into nitrite (NO₂⁻). Nitrite is equally toxic - it binds to haemoglobin and prevents fish blood from carrying oxygen. Fish suffering from nitrite poisoning often gasp at the surface even in well-oxygenated water, because the problem isn't dissolved oxygen, it's the blood's inability to use it.

Stage two: Nitrobacter bacteria (and other species in the Nitrospira genus, which are actually more dominant in established tanks than textbooks suggest) convert nitrite into nitrate (NO₃⁻). Nitrate is far less harmful and rises slowly. Plants consume some of it, but the main removal method is water changes.

A fully cycled tank keeps ammonia and nitrite at 0 ppm permanently. That's the benchmark - not "low", not "barely detectable". Zero.

Where the bacteria actually live

This is the piece most beginners misunderstand. Beneficial bacteria are not floating around in your water column in useful numbers. They're biofilm - thin layers of microorganisms attached to surfaces. The key requirement for a productive colony site is high oxygen and constant water flow, because these are aerobic bacteria: they need oxygen to do their job.

That makes your filter the most important real estate in the tank. Water is pumped through it continuously, delivering both oxygen and a constant supply of ammonia and nitrite for the bacteria to process. Inside the filter, porous media - ceramic rings, sintered glass, sponge - provide enormous surface area in a small volume. A handful of quality ceramic rings can have more colonisable surface than all the glass walls of your tank combined.

Secondary colonies establish on substrate, plant roots, decor, and the tank walls themselves. These matter, especially in heavily planted tanks, but they're not a substitute for a well-stocked filter.

Media types and what they're actually for

Mechanical media (coarse and fine sponges, filter wool) removes physical particles. Replace this when it's clogged beyond rinsing - the biological value is low because flow is poor when it's packed with debris.

Biological media (ceramic rings, sintered glass, bio balls, sponge with high surface area) is where your bacterial colony lives. Never replace the entire biological media volume at once. If you need to replace worn-out media, do it in stages over several weeks so the existing colony seeds the new material.

Chemical media (activated carbon, phosphate removers) targets specific dissolved compounds. Most tanks don't need it running permanently. Carbon in particular removes medications - remove it before treating disease.

Cycling a new tank: the right way to do it

A new tank has no bacteria. Add fish immediately and the ammonia from their waste will spike within days with nothing to process it. The correct approach is to establish the colony before adding fish - a process called fishless cycling. Full details are in the how to cycle a fish tank guide, but the principle is straightforward: dose ammonia into the empty tank, test daily, and wait until the bacteria population grows large enough to process it to zero.

Speeding up the process is genuinely possible. Adding a seeded sponge or a handful of used substrate from an established, disease-free tank can cut cycling time from weeks to days. Many fish shops will give you a small piece of old filter sponge if you ask - it's one of the most useful things you can take home from a pet store and it costs nothing.

Testing: what to measure and when

You need a liquid test kit. The API Master Test Kit is the most commonly recommended starting point, not because it's the only option, but because it covers ammonia, nitrite, nitrate, and pH in one box at a reasonable cost. Test strips exist, but they give ballpark readings that vary significantly between brands and even between strips in the same tube. When you're diagnosing whether 0.25 ppm ammonia is present, "somewhere between 0 and 0.5" isn't good enough.

During a cycle, test every day or two. In a newly established tank, test weekly for the first month. In a stable, long-running tank with a consistent stocking level, testing every two weeks is usually fine - though any time fish are behaving oddly, parameters should be the first thing you check. The aquarium water parameters guide covers safe ranges for the full suite of tests.

What disrupts an established cycle

A cycled tank can lose its bacterial colony faster than most people expect. The causes are usually one of the following:

Replacing all filter media at once. The single most common cause of a surprise ammonia spike in an established tank. If your instructions say "replace the cartridge monthly," ignore them. Rinse and reuse biological media unless it's physically falling apart.

Treating with medication. Many antibiotics and some antiparasitic treatments are harmful to bacteria. If you treat the main tank rather than a hospital tank, test parameters carefully for the week following treatment and be ready to do additional water changes.

Extended power cuts. Beneficial bacteria are aerobic - they need oxygen. A filter that's been off for several hours in a warm tank can start losing its colony. If power goes out for more than four to six hours, test your water when it's restored before assuming everything is fine.

Sudden large water changes with cold or unchlorinated water. Temperature shock and chlorine both damage bacteria. Use a dechlorinator (sodium thiosulfate-based products like Seachem Prime are widely used) and match water temperature to within a couple of degrees.

Overstocking rapidly. Adding a large number of fish at once spikes ammonia faster than the bacterial colony can adapt. Even a cycled tank has a capacity - the colony size is proportional to the bioload it's been processing. Add fish gradually and your bacteria will grow with the demand. The free stocking calculator helps you estimate bioload before you buy.

Managing nitrate long-term

Once your tank is cycled, ammonia and nitrite should stay at zero indefinitely. The compound that will rise over time is nitrate, and managing it is ongoing work rather than a one-time fix.

Most freshwater fish tolerate nitrate well up to around 20-40 ppm. Shrimp, sensitive species, and fish that spend their lives in fast-flowing rivers (many tetras, rasboras, hillstream loaches) prefer it below 20 ppm. Some fish - particularly livebearers and some cichlids - are more tolerant, but no fish does better with high nitrate than low nitrate.

Water changes are the primary tool. How often and how much depends on your stocking density, feeding habits, and plant coverage - the water change frequency guide goes into the specifics. Live plants help by consuming nitrate as a nutrient, but in a moderately stocked tank they rarely remove it fast enough on their own to skip water changes entirely.

If your nitrate is climbing rapidly between changes - say, more than 20 ppm per week in a moderately stocked tank - it usually points to overfeeding, overstocking, or insufficient water change volume rather than any problem with the cycle itself.

What most guides miss

Nitrospira vs. Nitrobacter: Most beginner resources attribute the nitrite-to-nitrate step entirely to Nitrobacter. Research over the past two decades has shown that Nitrospira species actually dominate this function in most aquarium biofilms, and some Nitrospira strains can perform both steps (complete ammonia oxidation, or "comammox"). It doesn't change how you manage your tank, but it explains why some bottled bacteria products that emphasise Nitrobacter underperform - they may not contain the organisms that matter most.

The cycle never fully "finishes": A cycled tank isn't a static system. The bacterial population fluctuates constantly in response to bioload, temperature, and available oxygen. A long-established tank handles minor disturbances well because the colony is large and resilient, but the biology is always dynamic. This is why gradual changes - in stocking, feeding, and temperature - are consistently better than sudden ones.

pH affects ammonia toxicity dramatically: At pH 7.0, the majority of ammonia in your water is actually ionised ammonium (NH₄⁺), which is far less toxic. At pH 8.0, the proportion of toxic free ammonia (NH₃) is much higher for the same total ammonia reading. This means an ammonia reading of 0.5 ppm at pH 6.8 is considerably less dangerous than the same reading at pH 8.2. It's not an excuse to ignore ammonia, but it's important context when you're looking at test results in a tank with naturally hard, alkaline water.