Nitrogen Cycle

For a high-level tour of how every element moves through the tank, start with Nutrient Cycling. This page follows nitrogen — the element that, more than any other, decides whether your aquarium thrives or slowly fades.

Why nitrogen matters most

Nitrogen is the currency of life in an aquarium. Proteins are the molecular machinery of every cell, every protein is built from nitrogen-bearing amino acids, and so an organism that runs out of usable nitrogen simply cannot grow — no matter how much light, carbon, or phosphorus surrounds it. The simulator leans into this: it measures every organism's biomass in terms of its nitrogen, and the most common cause of long-term decline in a closed tank is the slow exhaustion of available nitrogen.

What makes nitrogen uniquely precarious is that it is the only element with a permanent escape hatch. Denitrifying bacteria in anoxic sediment convert nitrate into nitrogen gas, which bubbles out of the water and is gone for good. Every other element — carbon, phosphorus, silicon, calcium — either stays put or trades reversibly with the atmosphere. Nitrogen leaks. In a sealed jar that slow leak is the single biggest threat to long-term survival, because the total nitrogen on hand sets a hard ceiling on how much life the system can carry.

Underneath all the biochemistry that follows, the story is simple: organisms build nitrogen into their bodies, death and decay release it again, and denitrification quietly bleeds a little away each day. Everything else is detail about how fast, and by which path.

The three dissolved forms

Nitrogen dissolved in the water exists in three inorganic forms, and the journey between them is the heart of the "cycle" hobbyists talk about:

Ammonium (NH4) is the preferred nitrogen source for almost every organism. It is energetically cheap to use because a cell can build it straight into protein without first having to chemically reduce it. It is also the most acutely toxic form to animals — the danger of an uncycled tank.
Nitrite (NO2) is a short-lived stepping stone produced halfway through nitrification. It is near zero in a mature tank but spikes during the establishment of a new one.
Nitrate (NO3) is the relatively benign end product. Algae can use it, but only by spending light energy to reduce it first (via the enzyme nitrate reductase), so they reach for ammonium first whenever both are available.

Together these make up the dissolved inorganic nitrogen (DIN) pool — the nitrogen that is immediately available for life to take up.

Where ammonium comes from

Ammonium is the hub of the whole cycle, fed by a surprising number of tributaries:

Animal excretion. Grazers like copepods and daphnia hold their bodies to a roughly fixed carbon-to-nitrogen ratio. When they eat food richer in nitrogen than they need, they dump the surplus straight back into the water as ammonium. The richer the food relative to their own composition, the more they excrete.

Fish. Fish are a special case and usually the dominant ammonia source in a stocked tank. Because their biomass is treated as a fixed stocking level rather than something that grows, there is nowhere for assimilated nitrogen to accumulate — essentially every nitrogen atom a fish takes in is excreted as ammonia, and everything it doesn't digest becomes feces, then detritus, then mineralized ammonia. This is why a live fish load drives cycling so much harder than a sprinkle of fish food. The nitrogen ultimately enters the tank with the food the keeper adds — the model's only external organic input — so the ammonia load tracks how much you feed: every gram of feed-nitrogen becomes ammonia, whether the fish eat it (and excrete it) or it sits uneaten and rots. Fish excrete ammonia in a form that nudges the water's buffering capacity upward at first, but as that ammonia is nitrified the net effect drains alkalinity — one of the quiet reasons a heavily stocked tank's pH tends to creep down over time.

Decomposition. When detritus breaks down, a portion of its nitrogen is released directly as ammonium (the rest passes through dissolved organic matter first). Heterotrophic bacteria feeding on dissolved organic matter (DOM) do the same balancing act animals do — when their food carries more nitrogen than they need, the excess comes out as ammonium. Sunlight also breaks down dissolved organics directly (photodegradation), releasing their nitrogen as ammonium during daylight.

The buried substrate. In a planted tank with an organic soil, decomposition of soil organic matter releases ammonium into the pore water — the interstitial water between soil grains. A fresh substrate also carries an initial pulse of dissolved ammonium that desorbs the moment it is submerged. This pore-water ammonium is the primary nitrogen source in a Walstad-style tank, standing in for bottled fertilizer (Walstad 1999). It reaches plants by two routes: roots draw it directly, and the rest slowly diffuses upward into the water column. Soil mineralization is far slower than decomposition in open water — a matter of fractions of a percent per day — but it is relentless and long-lived, which is exactly why a well-built soil tank can run for years without dosing.

Nitrogen fixation. Cyanobacteria are the only organisms in the model that can pull nitrogen gas out of the atmosphere and convert it into a usable form. This is the only process that adds genuinely new nitrogen to the system; everything else merely recycles what is already there. More on this below — it is the only patch for the denitrification leak.

Where ammonium goes

Producers take it up. Every alga and plant draws ammonium from the water, and most can also store a reserve of nitrogen internally when supply is good. This internal larder is the reason a healthy planted tank can coast through a week of low dissolved nitrogen without visibly slowing: cells keep dividing on their stored reserves for several days after the water itself looks empty, the same "starvation lag" seen in laboratory cultures and in real post-bloom lakes (Goldman & Glibert 1983). Rooted plants run two larders at once — one filled by their leaves from the water column, one filled by their roots from the pore water — and the plant grows on whichever is richer. That dual supply is why a soil-rooted plant keeps thriving when the water column is bare, and a water-column-fed plant keeps thriving when the substrate is spent. Cyanobacteria carry by far the largest nitrogen reserve of any producer, which is part of what makes them such stubborn late-bloom competitors.

In dim light, algae strongly prefer ammonium, because using nitrate requires light-derived energy to reduce it. In bright light they will take up nitrate as well.

Nitrifying bacteria convert it. This is the process at the centre of "cycling a tank," and it is important enough to get its own section below.

It escapes as gas (in open tanks, at high pH). A small fraction of total ammonia exists as dissolved ammonia gas, which can evaporate from an open water surface. This is a minor pathway in most freshwater tanks — at normal pH almost all ammonia is locked in the non-volatile ammonium form — but it becomes a real removal route above pH 9. See Dissolved Gases and Gas Exchange.

Nitrification: the cycle every aquarist waits for

Nitrification is the two-step bacterial conversion of toxic ammonium into relatively harmless nitrate, and watching it establish is the defining experience of starting a new tank. Three guilds of bacteria do the work:

Ammonia oxidizers convert ammonium to nitrite. This step does most of the heavy lifting — it consumes most of the oxygen and releases the acid that drains the water's buffer.
Nitrite oxidizers convert nitrite to nitrate, finishing the job.
Comammox bacteria (a strain of Nitrospira) do both steps inside a single cell, turning ammonium straight to nitrate with no nitrite ever leaving the cell.

Tracking these as separate guilds rather than one lumped reaction lets the simulator reproduce four things that aquarists actually observe:

The new-tank nitrite spike. In a fresh tank the ammonia oxidizers establish a little faster than the nitrite oxidizers, and the nitrite oxidizers can't even begin until there is nitrite for them to eat. So for a week or two the ammonia oxidizers run ahead, nitrite piles up, and then clears once the second guild catches up. This is the classic nitrite spike of weeks two and three — the universal signal that cycling is underway. Aquarium nitrite oxidizers are Nitrospira rather than the Nitrobacter of older textbooks (Hovanec 1998), and they behave like slow, steady specialists, which is exactly why the spike appears and then resolves.

Why mature tanks show no nitrite at all. Established filters come to be dominated by comammox Nitrospira (Bartelme et al. 2017; Sauder et al. 2017), because comammox have an enormously higher affinity for ammonium — they can scavenge it down to trace levels long after the faster ammonia oxidizers have run out of usable substrate. Since comammox skip the nitrite stage entirely, a mature tank oxidizes ammonium clean through to nitrate with no detectable nitrite. The shift from a nitrite-spiking new tank to a nitrite-free mature one is not scripted — it emerges from the bacteria competing for an ever-scarcer ammonium supply.

Cycling consumes oxygen and acid in a lopsided way. Three-quarters of the oxygen demand and essentially all of the acid release happen on the first (ammonia-oxidizing) step. So early in cycling, when that first guild dominates, the tank sees a sharper oxygen dip and pH drop than a simpler model would predict.

Nitrite is toxic too. Nitrite enters fish and crustaceans through the gills and sabotages their oxygen-carrying blood pigments. The model applies nitrite stress at species-specific thresholds — daphnia begin to suffer well before hardier snails do — capturing a hazard the older single-step view of cycling missed entirely.

Why cycles stall: pH and temperature

Two environmental controls govern the rate of nitrification, and both are behind the most common "my cycle won't finish" complaints.

pH controls the rate, not just survival. The ammonia-oxidizing enzyme works on dissolved ammonia gas, not on the ammonium ion — and the fraction present as ammonia gas falls roughly tenfold for every unit the pH drops. So in acidic water almost all the ammonia is in the form the enzyme cannot touch, and nitrification starves even when a test kit reads plenty of "ammonia." The literature is consistent (US EPA; Fritz Aquatics; university aquaponics extensions): nitrification runs best around pH 7.5–8.5, slows noticeably below 6.8, is badly inhibited below 6.5, and effectively stops near 6.0. This is the mechanism behind the classic "my cycle stalled when the pH crashed" failure — common in soft-water, planted, and blackwater tanks, and in any under-buffered tank where the cycle's own acid output eats through the carbonate buffer until the pH falls, the rate collapses, and the cycle arrests itself until a water change or buffer addition restores the alkalinity. (Importantly, this rate effect is separate from the extreme-pH stress that kills the bacteria outright; the cycle can stall long before the bacteria are in danger.)

Temperature controls the rate, with a sweet spot. Nitrification speeds up with warmth up to an optimum near 28–30 °C, then declines as the enzymes begin to fail, ceasing entirely in genuinely hot water. So a warm tank cycles a little faster than a cool one, but an overheated tank actually cycles slower — and because the bacteria's maintenance costs keep climbing with temperature even as their growth falls off, a too-hot tank can stall. This is why gentle warmth speeds a fishless cycle but a heatwave does not.

Nitrate and the two fates of the cycle's end product

Once nitrate forms, it has three possible fates. Algae and plants take it up (preferring ammonium when they can). And in the anoxic zones of the sediment, two competing guilds of anaerobic bacteria reduce it — but to very different ends.

Denitrification is the leak. Where oxygen has run out, denitrifying bacteria use nitrate in place of oxygen to breathe, oxidizing organic carbon and converting the nitrate to nitrogen gas that escapes the system forever. This is the one permanent nitrogen loss, and over weeks and months it is what slowly lowers a closed tank's total nitrogen and, with it, its carrying capacity. The system doesn't crash — it gently fades, each trophic level shrinking as the nitrogen base erodes beneath it.

How fast it leaks depends almost entirely on the sediment. Denitrification only happens where conditions are anoxic, and anoxic zones form where oxygen demand outpaces the supply diffusing down from the water. In a planted soil tank the soil layer is anoxic across most of its depth — its heavy mineralization demand strips the oxygen within millimetres of the surface — so denitrification runs steadily even beneath a well-oxygenated water column. In a bare-glass jar there is no anoxic zone at all, and almost no denitrification. In between, a thick layer of settled detritus on a fine-grained substrate develops large anoxic zones and loses nitrogen quickly, while a thin layer on coarse gravel barely loses any. Substrate choice is therefore one of the strongest levers on long-term nitrogen retention. Coupled to this, nitrate produced by nitrification within the thin oxic skin of the sediment sits right on the doorstep of the anoxic zone below, so it is an especially efficient denitrification substrate (coupled nitrification-denitrification; Nielsen 1992).

DNRA is the insurance policy. The same anoxic zones host a competing pathway, DNRA, which reduces nitrate all the way back to ammonium instead of to nitrogen gas — keeping the nitrogen in the system and immediately reusable. Which pathway wins comes down to the ratio of organic carbon to nitrate. DNRA is the more electron-hungry reaction, so when carbon is plentiful relative to nitrate it has the thermodynamic edge, letting its bacteria burn more of the surplus carbon per nitrate consumed. The organic-rich, nitrate-poor sediments of small sealed systems are exactly its preferred conditions. This produces a genuinely counterintuitive result: a tank with more organic matter on the bottom can retain nitrogen better, because the high carbon-to-nitrate ratio steers anaerobic metabolism toward nitrogen-conserving DNRA and away from nitrogen-losing denitrification.

Both pathways also nudge alkalinity upward, partly offsetting the buffer that nitrification consumed — which is why a tank with healthy sediment biology resists the slow downward pH drift better than a bare one.

The rhizosphere: how roots reshape the sediment

Rooted plants are not passive drinkers of pore water — they actively re-engineer the chemistry around their roots by leaking a little photosynthetic oxygen down through internal air channels (aerenchyma) and out into the otherwise anoxic substrate. This radial oxygen loss creates a thin oxidized film on every root surface, and that film does several jobs at once. Together they are a large part of why a heavily-planted soil tank stays stable where a bare organic substrate would turn sour.

The most important effect is a tight little nitrogen pump. In the oxic film, ammonia oxidizers convert pore-water ammonium to nitrate; the nitrate then diffuses a fraction of a millimetre into the surrounding anoxic pore water and is denitrified to nitrogen gas. This coupled rhizosphere process is the dominant nitrogen-export route in real rooted wetlands — field studies attribute the majority of total nitrogen loss to it (Reddy et al. 1989), far more than water-column denitrification manages on its own. The same oxic film also oxidizes toxic sulfide before it can reach harmful levels (protecting roots from the substrate, Lamers et al. 2013) and precipitates dissolved iron as the rust-coloured "iron plaque" seen on wetland roots, which drags phosphate down with it. The whole apparatus runs on oxygen the plant supplies from its own photosynthesis, so it is only present where living roots are present — fast strap-leaved plants like Vallisneria contribute more of it than slow rosette plants like Cryptocoryne.

Adding nitrogen back: cyanobacterial fixation

If denitrification is the leak, nitrogen fixation by cyanobacteria is the only patch. Cyanobacteria alone can convert atmospheric nitrogen gas into usable ammonium inside their own cells — but the machinery, the nitrogenase enzyme, is expensive and fussy, so fixation is tightly regulated by four conditions:

Light, because nitrogenase runs on energy from photosynthesis — fixation only happens by day.
Internal nitrogen status. A cyanobacterium senses its own cellular nitrogen reserve, not the concentration in the water. While its reserve is full, fixation stays switched off; only when the reserve drains below about half does it switch on. The practical consequence is that cyanobacteria keep fixing for several days into a nitrogen crash — long after the water has gone empty — which makes them robust competitors precisely when every other producer is starving (Allen 1984; Lindblad & Bergman 1986).
Oxygen, because nitrogenase is destroyed by it. Fixation is fastest when dissolved oxygen is low.
Phosphorus, because fixation is ribosome- and energy-hungry. A phosphorus-starved cyanobacterium cannot fix effectively.

Because fixation diverts a large slice of the cell's energy budget, cyanobacteria are poor competitors when nitrogen is plentiful — but they come into their own when nitrogen runs out and everything else is starving. In a long-running sealed system, the endgame often turns on whether cyanobacteria can fix nitrogen fast enough to offset the steady denitrification drain.

Iron and molybdenum: the master switches

Almost every step that moves nitrogen between its inorganic forms is run by an enzyme built around a metal cofactor — and the two metals are iron and molybdenum.

Iron sits at the heart of nitrogenase (fixation), the ammonia-oxidizing enzyme (nitrification), and the nitrate-reducing enzymes (denitrification), as well as the chlorophyll and electron-transport machinery of photosynthesis itself. Iron scarcity therefore pinches the entire nitrogen cycle at once, and in a predictable order: fixation falters first (diazotrophs are especially iron-hungry), then nitrification and denitrification. In iron-poor water — RO-based systems, or hard alkaline water where iron precipitates the moment it meets oxygen — this creates an unsettling regime in which denitrification keeps draining the tank but cyanobacteria can't fix nitrogen to replace the loss. Sediment-dwelling denitrifiers partly sidestep the problem by tapping iron from the substrate's mineral reserves. See the Iron Cycle for the full picture.

Molybdenum is the second, smaller switch, sitting in series with iron on the two reductive steps. It is the cofactor of nitrate reductase (used by every producer that takes up nitrate) and a component of nitrogenase. Because neither enzyme has a molybdenum-free substitute, molybdenum scarcity shows up in two specific ways: producers lean harder on ammonium and away from nitrate, and cyanobacterial fixation hits a ceiling independent of iron. The molybdenum ceiling can bite first in very soft water, even in a tank where iron has been dosed to abundance. See Micronutrient Cycling.

A note on alkalinity

Each nitrogen transformation tugs on the water's alkalinity, and the net effect over a tank's life is what drives slow pH drift. Ammonium uptake by plants and nitrification both consume alkalinity (acidifying), while nitrate uptake, denitrification, DNRA, and the release of ammonium from decomposition all restore it. Nitrification is the big consumer — fully oxidizing ammonium to nitrate consumes about two units of buffer per nitrogen atom — which is why a heavily cycling, lightly buffered tank tends to acidify and, eventually, to stall its own cycle. A tank with active sediment biology and healthy plant uptake claws some of that buffer back. The exact stoichiometric bookkeeping for every reaction is tabulated in the Parameter Reference; the Carbonate System explains how those alkalinity changes translate into pH.