1. Ammonia
Produced from waste and decaying organic matter. It should not remain detectable in an established aquarium.
Aquarium basics
Cycling an Aquarium
A new aquarium can look clean long before it is biologically ready. Cycling gives beneficial bacteria time to establish so waste can be processed safely before fish, shrimp or other livestock are added.
The short version
What cycling actually means.
Fish waste, uneaten food and other organic material eventually produce ammonia. Ammonia is toxic. In a cycled aquarium, colonies of beneficial bacteria convert ammonia into nitrite, and then another group converts nitrite into nitrate.
2. Nitrite
Produced as ammonia is processed. Nitrite is also toxic and should read zero once the aquarium is fully cycled.
3. Nitrate
The end product of the basic aquarium nitrogen cycle. Nitrate is managed through plants, maintenance and water changes.
The useful way to think about it:
cycling is not “waiting a few weeks”. It is establishing a working
biological filter and confirming that it can process waste.
Before you start
Set the aquarium up properly first.
Install the filter, fill the aquarium with dechlorinated water and run the system normally. If the aquarium will be heated, run the heater too. Beneficial bacteria mainly colonise surfaces, especially filter media, rather than simply floating around in the water.
- Use dechlorinated water.
- Keep the filter running continuously.
- Run the aquarium at the temperature you plan to keep it.
- Add plants and hardscape if they are part of the final setup.
- Have a reliable ammonia, nitrite and nitrate test available.
Fishless cycling
Give the bacteria a controlled ammonia source.
A fishless cycle is preferable because livestock are not exposed to rising ammonia or nitrite while the bacterial colonies develop. The aquarium needs a source of ammonia so the bacteria have something to process.
This can be done with a measured aquarium-safe ammonia source or, less precisely, with a small amount of decomposing food. The exact method matters less than being able to monitor the water and avoid adding so much organic matter that the aquarium becomes a mess.
Avoid cycling with livestock on purpose.
A fish-in cycle can expose animals to toxic ammonia and nitrite.
If animals are already in an uncycled aquarium, the priority becomes
keeping those compounds as close to zero as possible through
testing, water changes and careful feeding.
What you should see
The readings change as the cycle develops.
Early on, ammonia may be present while nitrite remains at zero. As the first bacterial population develops, ammonia begins to fall and nitrite appears. Later, nitrite also begins to fall as nitrate accumulates.
Real aquariums do not always follow a perfectly neat graph. Plants, seeded filter media, temperature, pH and the amount of ammonia being added can all change how quickly the process appears to move.
When is it ready?
Do not judge it by the calendar.
A cycle is considered established when the aquarium can process the ammonia being added without leaving measurable ammonia or nitrite behind. Nitrate is usually present unless plants are consuming a substantial amount of it.
Ammonia
0 ppm
Nitrite
0 ppm
Nitrate
Usually detectable
Before adding livestock, perform any needed water change to bring nitrate down to a sensible level and make sure temperature and other parameters suit the animals you plan to keep.
Common mistakes
Things that slow people down.
- Adding animals simply because the tank has been running for a certain number of days.
- Replacing all filter media and accidentally removing a large part of the bacterial colony.
- Washing biological filter media under chlorinated tap water.
- Adding too much food during a fishless cycle and creating unnecessary decay.
- Testing once, seeing zero ammonia, and assuming the cycle is complete without checking nitrite.
- Adding a full stock of fish immediately instead of increasing the biological load gradually.
After cycling
The bacteria still depend on the way you maintain the tank.
Cycling is not something the aquarium finishes and then forgets. The bacterial population responds to the amount of waste being produced. Sudden overstocking, heavy feeding, medication, filter failure or aggressive cleaning can all affect biological filtration.
Add livestock sensibly, feed appropriately, keep the filter running and continue testing whenever something seems wrong.
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