How to cycle a new aquarium

What cycling is and why it matters

A new aquarium is clean, but it is not yet safe. Fish, shrimp and snails constantly produce waste, and leftover food and dead plant parts decay. All of this releases ammonia (NH3), which is toxic to aquatic animals even in small amounts.

In an established aquarium, bacteria remove that ammonia as fast as it is produced. A new tank does not have these bacteria yet. Cycling is the period in which you let them grow – in the filter, on the substrate and on every surface – before the tank has to support animals.

The nitrogen cycle in three steps

1. Ammonia (NH3) appears. It comes from waste and from anything that decays in the water. It is toxic.
2. Bacteria convert ammonia into nitrite (NO2). Nitrite is also toxic, so this is progress, not the finish line.
3. A second group of bacteria converts nitrite into nitrate (NO3). Nitrate is far less harmful and is removed by regular water changes and used by plants.

During cycling you can watch these steps in your test results: first ammonia rises, then nitrite rises while ammonia falls, and finally nitrate appears while nitrite falls back to zero.

How to cycle a new aquarium, step by step

1. Set up the tank completely: substrate, hardscape, plants, filter and heater if your animals need one. Fill it and treat tap water with a water conditioner.
2. Start the filter and keep it running all the time. The bacteria live mainly in the filter media and need a constant flow of water.
3. Give the bacteria something to eat. Without animals in the tank, the bacteria need another source of ammonia – for example a small pinch of fish food that decays, or a bottled ammonia product made for cycling, used as described on its label.
4. Measure ammonia (NH3), nitrite (NO2) and nitrate (NO3) regularly and write the results down. A record is the only way to see where in the cycle your tank is.
5. Wait. Do not add animals because the water looks clear. Clear water says nothing about ammonia or nitrite.

When is cycling finished?

This is the question most beginners get wrong, usually by stopping too early. Tank Manager uses a clear rule, and all three conditions must be true at the same time:

• At least 28 days have passed since cycling started.
• In the last 7 days there are at least 3 measurements, and every one of them shows ammonia (NH3) = 0 and nitrite (NO2) = 0.
• Nitrate (NO3) is measurable. Zero ammonia and zero nitrite without any nitrate usually means the cycle never really started, not that it is finished.

If any of the three conditions is missing, keep cycling and keep measuring. A cycle that takes longer than 28 days is normal – 28 days is the minimum, not a deadline.

Common mistakes

• Adding animals after a few days because the water is clear.
• Measuring once, seeing zero, and calling it done. One measurement is not a trend.
• Switching the filter off for long periods. The bacteria need flow.
• Washing filter media under tap water or replacing all of it during cycling. That removes the bacteria you are trying to grow.
• Not measuring nitrate. Without nitrate you cannot tell a finished cycle from one that has not started.

After cycling

A finished cycle means the tank can handle a small, gradual start – not a full stock on day one. The bacteria colony is only as large as the amount of waste it has been processing. Add animals in stages and keep measuring; the next guide covers the first three months.