Cycling your tank
Before you add fish, you need an invisible army of beneficial bacteria living in the filter and substrate. That process is called cycling. Skip it and ammonia from waste will burn gills and organs — what hobbyists call New Tank Syndrome.
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What you will need
Test kits first · Then conditioner and bacteria
You cannot cycle a tank by guesswork. A liquid test kit is essential. Conditioner protects the bacteria from tap water, and bottled bacteria can speed things up.
Test kits
Water conditioner
Bottled bacteria
Ammonia source (fishless cycle)
Why your tank needs to cycle
Ammonia is the silent killer · Bacteria make it safe
In a closed aquarium, fish waste, uneaten food and decaying plants break down into ammonia (NH3 / NH4+). Ammonia is highly toxic. The nitrogen cycle is the natural process that turns it into safer compounds, one bacterial colony at a time.
Those bacteria live mainly on filter media, rock and sand — not floating in the water. Protect the filter and you protect the cycle.
The three stages
Waste → ammonia → nitrite → nitrate
The cycle happens in three phases as different bacteria establish themselves.
Waste and food decay into ammonia. This is the first reading you will see on a test kit.
Nitrosomonas bacteria convert ammonia into nitrite. Still toxic, but it means the first colony is working.
Nitrospira / Nitrobacter convert nitrite into nitrate. Safe in modest amounts and removed with water changes or plants.
| Stage | Process | Key player | Toxicity |
|---|---|---|---|
| Stage 1 | Waste converts to ammonia | Organic decay | High (lethal) |
| Stage 2 | Ammonia converts to nitrite | Nitrosomonas | High (lethal) |
| Stage 3 | Nitrite converts to nitrate | Nitrospira / Nitrobacter | Low (safe in modest amounts) |
Fishless cycle: step by step
No fish in the tank · Usually 4–8 weeks
You add a source of ammonia to feed the bacteria. That builds the colony without putting livestock at risk. Bottled bacteria or a squeeze of established filter media from a healthy tank can shorten the wait, but you still confirm with tests.
Set up and dechlorinate
Fill the tank. Run the filter and heater. Set the heater around 26–28°C while cycling — bacteria grow faster when warm. You can lower it later for the livestock you keep.
Crucial: treat tap water with a conditioner. Chlorine and chloramine kill beneficial bacteria on contact.
Add an ammonia source
You need to “feed” the bacteria. Use one method, not both:
- Pure ammonium chloride (preferred): dose until the test kit reads 2.0–4.0 ppm ammonia. Follow the bottle for your tank volume.
- Fish food method: a small pinch of flake food each day. As it decays it releases ammonia. Slower and harder to control than bottled ammonia.
Test every 2–3 days
Use a liquid test kit for ammonia, nitrite and nitrate (strips are less reliable). After a week or two, ammonia should start to drop and nitrite should start to rise. That is the first colony at work.
Ride the nitrite peak
Keep a small ammonia dose in the tank (around 1 ppm) so the first colony does not starve. Nitrite will climb, then fall as the second colony forms. This is often the longest wait.
Confirm the finish line
Dose ammonia, then test 24 hours later. The tank is cycled when you get the readings below. Then do a large water change before any fish go in.
When it is done
24 hours after an ammonia dose · Then a water change
You are looking for ammonia and nitrite at zero, with nitrate showing that the last step is working.
Critical tips
Protect the bacteria · They live in the filter
Don’t over-clean
Never rinse filter media in tap water. Squeeze it in a bucket of old tank water during a change. Chlorine will wipe out the colony you just grew.
Oxygen matters
These bacteria need oxygen. Keep the filter running and make sure there is surface movement, or add an airstone.
Need help?
Bring a water sample in and we can test it for you. A liquid kit, dechlorinator and bottled bacteria will get a new tank started on the right foot.
