The Nitrogen Cycle, Explained Simply
If you've just set up a new aquarium, this is the single most important thing to understand before adding fish. The nitrogen cycle is the invisible biological process that keeps your water safe — and skipping it is the number one reason new aquariums lose fish in the first few weeks.
What Is the Nitrogen Cycle?
Every living thing in your aquarium — fish, shrimp, snails, even leftover food — produces waste. That waste breaks down into ammonia, a colourless chemical that is highly toxic to fish even in tiny amounts. In the wild, ammonia is constantly diluted by huge volumes of water and processed by naturally occurring bacteria. In a glass box with a fixed amount of water, it has nowhere to go — unless you build that same bacterial workforce into your filter first.
The nitrogen cycle is simply the process of growing a colony of beneficial bacteria in your filter media, gravel, and other surfaces. These bacteria convert dangerous ammonia into a slightly less toxic chemical called nitrite, and a second group of bacteria then converts that nitrite into nitrate — a compound that's relatively safe for fish at low concentrations and easily removed through regular water changes.
The Cycle, Visualised
Where does the bacteria actually live?
Beneficial bacteria don't float free in the water — they colonise surfaces:
- Filter media (sponges, ceramic rings, bio-balls) — the main bacterial "home"
- Gravel and substrate surfaces throughout the tank
- Driftwood, rock, and any other submerged hardscape
Ammonia is converted to nitrite by one group of bacteria (genus Nitrosomonas), then nitrite is converted to nitrate by a second group (genus Nitrobacter). Nitrate is far less toxic and is managed with regular water changes or absorbed by live plants.
What Happens Week by Week
Tank Set Up, Ammonia Source Added Start
Fill and set up the tank with substrate, hardscape and filter running. Add an ammonia source — pure ammonia solution, a piece of raw prawn, or fish food — to feed the bacteria you're trying to grow. No fish yet.
Ammonia Rises
Ammonia levels climb as waste breaks down with no bacteria yet present to process it. This is expected and normal — test daily and watch the number climb, then start to fall as bacteria begin to establish.
Nitrite Spikes
As ammonia-eating bacteria establish, ammonia starts to fall — but nitrite rises sharply as a second bacteria colony hasn't caught up yet. This "nitrite spike" is usually the longest, most stubborn stage of the cycle.
Nitrate Appears, Cycle Completes
Nitrite-eating bacteria catch up, nitrite falls to zero, and nitrate begins to accumulate instead. Once both ammonia and nitrite consistently test at zero within 24 hours of dosing, your tank is fully cycled.
Safe to Stock Finish
Do a large water change to bring nitrate down, then begin adding fish gradually — a few at a time rather than all at once — to avoid overwhelming your new bacterial colony.
Testing & What the Numbers Mean
Target: 0 ppm (with fish)
Safe at zero. Anything above 0.25 ppm is stressful to fish, and levels above 1–2 ppm can cause visible distress or death within hours. During fishless cycling, dosing ammonia up to 2–4 ppm is normal and expected.
Target: 0 ppm (with fish)
Just as dangerous as ammonia to livestock, nitrite interferes with a fish's ability to carry oxygen in its blood. This is usually the slowest number to reach zero during cycling — patience here is essential.
Target: Under ~20–40 ppm
The "end product" of the cycle and far less toxic — but not harmless at high concentrations. Keep it in check with regular partial water changes, live plants, or both, rather than letting it climb indefinitely.
How to Cycle a New Tank
Set Up Fully First
Add substrate, hardscape, water conditioner, and get the filter running continuously before adding any ammonia source. The filter needs constant flow and oxygen for bacteria to establish.
Add an Ammonia Source
Dose pure ammonia (no surfactants or perfumes) to around 2–4 ppm, or use a decaying shrimp/prawn or a pinch of fish food as a slower-release natural source.
Test Every 2–3 Days
Use a liquid test kit (more accurate than test strips) to track ammonia, nitrite and nitrate over time, and watch the pattern described in the timeline above unfold.
Keep Dosing Ammonia
Every time ammonia drops to zero, add more to keep feeding the growing bacterial colony — don't let the food source run out partway through the process.
Wait for Zero & Zero
The tank is cycled once both ammonia and nitrite read zero within 24 hours of a dose, while nitrate is clearly rising — proof both bacteria colonies are fully established.
Water Change, Then Stock Slowly
Do a large water change to lower nitrate before adding fish, then stock gradually over several weeks rather than filling the tank all at once.
Did You Know
You Can "Borrow" a Cycle
Adding a piece of filter media, some gravel, or even a cup of water from an already-established, healthy tank can seed a new tank with bacteria and cut weeks off the cycling process.
Rinsing Media in Tap Water Kills Bacteria
Chlorine and chloramine in tap water will kill your beneficial bacteria colony almost instantly — always rinse filter media in old tank water or dechlorinated water instead.
Live Plants Help From Day One
Fast-growing live plants absorb ammonia directly, which can meaningfully reduce ammonia spikes and shorten the overall cycling timeline in a heavily planted tank.
Bottled Bacteria Products Can Genuinely Help
Quality live bacterial starter products can speed up cycling significantly when used correctly, though they're not a substitute for testing and patience — read the product instructions carefully.
The Cycle Never Actually Stops
Even in a fully mature tank, this same bacterial process runs continuously in the background every single day — cycling isn't a one-time event, it's an ongoing biological system you're maintaining.
A "Mini Cycle" Can Happen Later Too
Adding a lot of new fish at once, a big filter clean, or antibiotic treatment can all knock back your bacteria colony temporarily — watch parameters closely after any of these events, even in an established tank.
Common Beginner Mistakes
Adding all your fish on day one
The single biggest cause of new tank syndrome. Stock gradually, whether cycling with fish or without, to avoid overwhelming a young bacterial colony.
Cleaning the filter media too thoroughly
Scrubbing filter sponges under tap water, or replacing all media at once, wipes out most of your bacteria colony — rinse gently in old tank water instead, and stagger media replacement.
Giving up on the cycle too early
Impatience is understandable, but adding fish before ammonia and nitrite both read zero simply moves the toxic exposure onto living animals instead of a test kit.
Turning the filter off overnight
Beneficial bacteria need constant oxygen flow to survive — even a few hours without water movement through the media can seriously set back an establishing colony.
Not testing at all
Without a liquid test kit, there's no way to actually know whether a tank is cycled — "it looks clear" tells you nothing about ammonia or nitrite levels, which are invisible to the eye.
Setting Up Your First Tank?
ECAAC members are always happy to help new hobbyists interpret test results, troubleshoot a stubborn cycle, or share a bit of established filter media to help a new tank along. Come along to a meeting and ask.