In short
- Feed lightly – less food means less ammonia.
- Measure ammonia (NH3) and nitrite (NO2) every day.
- When NH3 or NO2 are elevated, do smaller and more frequent water changes.
- Do not add any new animals until cycling is finished.
What fish-in cycling means
In a new aquarium the bacteria that remove ammonia and nitrite are not established yet. If fish are already living in it, their waste produces ammonia from the first day, and there is nothing in the tank to process it. The nitrogen cycle still develops – but this time the fish are exposed to it while it happens.
You cannot skip the cycle. What you can do is protect the animals during it. The four rules below are what Tank Manager recommends in this situation.
Rule 1: Feed lightly
Everything you feed ends up as ammonia – either through the fish or as leftovers decaying on the bottom. Feed small portions that are eaten completely, and remove anything that is left. Healthy fish cope with light feeding far better than with ammonia.
Rule 2: Measure every day
Measure ammonia (NH3) and nitrite (NO2) daily and write the values down. In an uncycled tank the values can change quickly, and a daily record is the only way to react in time. Measure nitrate (NO3) as well – its appearance tells you the bacteria have started working.
Rule 3: Smaller, more frequent water changes
When ammonia or nitrite are elevated, dilute them with a water change. Smaller changes done more often are better than one very large change: they bring the values down without a sudden shift in temperature and water chemistry, which would be an extra stress for fish that are already under pressure.
- Always treat new tap water with a water conditioner.
- Match the temperature of the new water to the tank as closely as you can.
- Measure again after the change, and repeat if the values are still elevated.
Rule 4: Do not add new animals
Every new animal adds waste to a tank that already cannot process what it has. Do not add fish, shrimp or snails until cycling is finished – no matter how empty the aquarium looks.
What not to do
- Do not replace or wash the filter media under tap water. The filter is where the bacteria are settling.
- Do not switch the filter off.
- Do not stop measuring because the fish look fine. Damage from ammonia and nitrite is often not visible at first.
When is fish-in cycling finished?
The same rule applies as for a tank without fish: at least 28 days since the start, at least 3 measurements in the last 7 days with ammonia (NH3) = 0 and nitrite (NO2) = 0, and measurable nitrate (NO3). Until then, keep following all four rules.
Frequently asked questions
I already have fish in a new tank. What should I do first?
Measure ammonia (NH3) and nitrite (NO2) today. Then feed lightly, measure every day, do smaller and more frequent water changes whenever the values are elevated, and do not add any new animals.
How often should I change water during fish-in cycling?
As often as your measurements require. Whenever ammonia or nitrite are elevated, do a smaller water change and measure again. Smaller, more frequent changes are safer than one large change.
Should I stop feeding my fish during fish-in cycling?
Feed lightly rather than normally. Small portions that are eaten completely keep ammonia production low.
Can I add more fish to help the cycle along?
No. More animals mean more ammonia in a tank that cannot process it yet. Wait until cycling is finished.
How long does fish-in cycling take?
At least 28 days. It is finished when at least 3 measurements in the last 7 days show ammonia (NH3) = 0 and nitrite (NO2) = 0 and nitrate (NO3) is measurable.
This guide is general information for aquarium keepers. It does not replace advice from a veterinarian or other qualified specialist for a specific problem. Last updated: 2026-11-14.