The nitrogen cycle explained
Ammonia turns into nitrite turns into nitrate. Here's what's actually happening in your filter and why it matters.
A new fish tank can kill fish faster than an old one. The reason is the nitrogen cycle, and most beginner problems come from not understanding it.
This piece covers what's happening biologically, how to know your tank is cycled, and what to do when something goes wrong.
What ammonia is and where it comes from
Fish breathe out ammonia. They also pee it out, and uneaten food rots into it. So does dead plant matter, dead fish, and pretty much anything organic that breaks down in water.
Ammonia is poisonous to fish at very low levels. Even 0.25 parts per million causes stress. At 1 part per million it starts doing real gill damage. Above 2 parts per million and you're looking at deaths.
In a brand new tank with no biological filtration, ammonia just accumulates. This is why "new tank syndrome" exists and why dropping fish into uncycled water is a slow kill.
The bacteria do the work
Two groups of bacteria handle this:
- Nitrosomonas (and similar species) eat ammonia and excrete nitrite
- Nitrobacter and Nitrospira eat nitrite and excrete nitrate
Nitrite is also poisonous to fish (it binds to hemoglobin and stops oxygen transport, the fish version of carbon monoxide poisoning). Nitrate is only mildly toxic and accumulates slowly. You remove it with water changes or, in a planted tank, plants take it up directly.
The full chain looks like:
fish waste -> ammonia (NH3) -> nitrite (NO2) -> nitrate (NO3) -> water change
A "cycled" tank means you've grown enough of both bacteria types that they keep up with the fish load. Ammonia and nitrite should both read zero. Nitrate slowly climbs until you change water.
How long it takes
Four to eight weeks for most tanks. Faster with bottled bacteria (Tetra SafeStart, Seachem Stability, Dr. Tim's). Slower in cold water. Stalls completely if pH drops below about 6.
The bacteria live mostly in your filter media, not your water. This is why you never replace all your filter media at once and why filter cartridges that say "replace monthly" are scammy. Rinse, don't replace.
Fishless cycling
The cleanest way to cycle a new tank: add ammonia (Dr. Tim's Ammonium Chloride works) to about 2 parts per million, wait for nitrite to spike then drop, keep adding until ammonia and nitrite both convert to zero within 24 hours of a fresh addition. Then water change down to safe nitrate levels and add fish.
Takes about a month with patience. No fish suffering.
Fish-in cycling
If you already have fish in there (you bought a tank kit and the store sold you fish the same day, this happens constantly), you cycle with the fish in. This means:
- Test ammonia and nitrite daily
- Water change immediately when either climbs above 0.25 parts per million
- Don't feed heavily
- Use a binder like Prime to detoxify ammonia between water changes
It works, but it's stressful for the fish.
Knowing it's cycled
A test kit (API Master Test Kit is the standard) showing ammonia 0, nitrite 0, nitrate detectable. The nitrate is the proof. Without that final number climbing, you haven't actually grown the second bacteria group yet.
Liquid test kits, not strips. Strips lie.
What breaks a cycle
- Replacing all filter media at once
- Bleaching your filter
- Running medications that kill bacteria (most antibiotics will)
- Letting the filter run dry for hours
- Big swings in pH
If you nuke your cycle, you go back to mini-cycling: small ammonia spikes, frequent water changes, a couple weeks to rebuild. Less drama than the first cycle because the rest of the tank already has bacteria seeded in the substrate and decor.
Why this matters for planted tanks and aquaponics
In a planted tank, plants take up ammonia directly (they prefer it to nitrate). Heavily planted tanks with low fish loads can run with very small bacterial colonies because the plants get to the ammonia first.
In aquaponics, the whole point is that nitrate feeds your plants. You want a fully cycled system so you have a steady nitrate supply. Aquaponics also uses a third group, the nitrate-reducing bacteria that turn excess nitrate into nitrogen gas. Most home systems don't get this third stage going.
What the numbers actually mean
When you test your water during and after cycling, here's how to interpret the results:
Ammonia above 0.25 parts per million in an established tank: Something is producing ammonia faster than your bacteria can process it. Overfeeding, a dead fish hidden behind decor, a filter that was cleaned too aggressively (killing bacteria), or a sudden addition of too many fish. Reduce feeding, find and remove any dead organisms, and do a 25% water change.
Nitrite above 0 parts per million in an established tank: The second stage of bacteria isn't keeping up. This often follows an ammonia spike by 2-3 days (it takes time for excess ammonia to be partially processed into nitrite that then overwhelms the second-stage bacteria). Increase aeration (these bacteria need oxygen), reduce feeding, and do water changes to keep nitrite below 1 part per million until the colony catches up.
Nitrate above 40 parts per million: Time for a water change. Nitrate is the end product and only leaves the system through water changes or plant absorption. In planted tanks, fast-growing plants absorb significant nitrate, but in fish-only setups, water changes are the only removal mechanism.
Nitrate at 0 parts per million in a stocked tank: Either the test kit is expired, the tank is heavily planted (plants are consuming all the nitrate), or the cycle has stalled (bacteria aren't processing ammonia at all, so no nitrate is being produced). Test ammonia and nitrite to differentiate.
Why the cycle crashes
An established cycle can crash (bacteria population drops dramatically) from a few causes:
Chlorine or chloramine exposure. Adding untreated tap water kills nitrifying bacteria. Always dechlorinate replacement water. This is the most common cause of sudden ammonia spikes in established tanks.
Medication. Some fish medications (antibiotics especially) kill beneficial bacteria along with the target pathogen. If you need to medicate, do it in a separate hospital tank when possible, not in the main display tank with the established biofilter.
Cleaning the filter too aggressively. Rinsing filter media in tap water (chlorinated) or replacing all the media at once removes the bacterial colony. Rinse filter media in old tank water during water changes. If replacing media, do half at a time and wait 2-3 weeks before replacing the other half.
Extended power outage. Without water flow through the filter, the bacteria in the filter media run out of oxygen and ammonia within hours. The population declines. After a long outage, monitor ammonia and nitrite daily for a week, as the cycle may need to partially re-establish.
The tank volume calculator helps you determine your tank's actual water volume (accounting for substrate, hardscape, and displacement) for accurate dosing during cycling.
The cycle in planted tanks
Plants add a third pathway for nitrogen removal. In addition to the bacterial nitrification chain (ammonia to nitrite to nitrate), plants absorb nitrogen directly. Most aquatic plants prefer ammonium (NH4+) over nitrate (NO3-) because ammonium requires less metabolic energy to assimilate. In a heavily planted tank, plants may absorb ammonia fast enough that the bacterial colony never needs to be very large.
This creates the possibility of a "silent cycle" in a new planted tank: if the tank is densely planted with fast-growing species (hornwort, water sprite, floating plants like frogbit or duckweed), you can add a small number of fish from day one. The plants absorb the ammonia before it reaches toxic levels. The bacterial colony grows gradually in the background. Ammonia and nitrite may never spike to measurable levels because the plants are consuming the ammonia before bacteria get to it.
The silent cycle works only with truly heavy planting and a very conservative initial fish load (2-3 small fish in a 100+ liter tank). It's not an excuse to skip cycling; it's a specific technique that requires more plants than most beginners install. If the plants can't keep up (because there aren't enough of them, or they're slow growers like anubias), ammonia accumulates and the fish suffer.
Denitrification: closing the loop
Standard aquarium filtration handles nitrification (ammonia to nitrate) but doesn't address denitrification (nitrate to nitrogen gas). In nature, anaerobic zones in deep sediment host denitrifying bacteria that convert nitrate back to atmospheric nitrogen, completing the nitrogen cycle.
In a standard aquarium, there are no significant anaerobic zones (the filter and substrate are aerobic by design), so nitrate accumulates and water changes are the only removal method. Some advanced setups include anaerobic denitrification chambers (a slow-flow, low-oxygen compartment in the filter path) that host denitrifying bacteria. These can reduce or eliminate the need for water changes to control nitrate, but they're complex to set up and maintain. For most hobbyists, regular water changes are simpler and more reliable.
Deep sand beds (10+ cm of fine sand) develop natural anaerobic zones in the lower layers that support some denitrification. This is more common in marine aquariums but can occur in freshwater tanks with deep fine-grained substrates. The risk: if the anaerobic zone is disturbed (by digging fish, deep cleaning, or uprooting plants), it can release hydrogen sulfide, which is toxic to fish in even small concentrations. Deep sand beds are a valid approach but require a hands-off management style for the lower substrate layers.
The tank volume calculator helps you determine your tank's actual water volume for accurate dosing during cycling.