How long to run a filter during a fish-in cycle

During a fish-in cycle, run your aquarium filter 24 hours a day, every day. Keep it running after the cycle is complete, too. The filter’s media provides a home for beneficial bacteria, and continuous water flow supplies the oxygen they need to process fish waste.

Run the filter continuously

Turning the filter off overnight or for long stretches can reduce water flow and oxygen around the media. That can slow the development of the bacterial colony your aquarium needs. Clear-looking water does not mean the cycle is complete; ammonia and nitrite can be present even when the water looks clean.

You can switch the filter off briefly for maintenance or feeding if necessary, but restart it as soon as you’re done. Follow the filter manufacturer’s instructions for priming and restarting it.

Protect the bacteria in the filter

• Keep the media wet. If you remove it for maintenance, keep it in aquarium water rather than letting it dry out.
• Rinse gently when needed. If debris is clogging the media, swish it in water removed from the aquarium during a water change. Avoid rinsing biological media under untreated tap water.
• Avoid replacing all the media at once. Replacing or deep-cleaning everything can remove much of the beneficial bacteria. Follow the filter maker’s replacement instructions, and replace media gradually when possible.
• Maintain good flow. Check that the intake and outlet are clear and that the filter is assembled correctly. Don’t let a clogged filter run without adequate water flow.

Monitor the fish-in cycle

Test the water for ammonia and nitrite regularly; checking daily is a practical approach during an active fish-in cycle. Use partial water changes with dechlorinated water as needed to keep fish safe, and avoid adding more fish until the cycle is established. A filter does not instantly remove ammonia, so don’t rely on it instead of testing and water changes.

If you use medication, read its label and the filter instructions. Some treatments call for removing particular chemical media, such as activated carbon, but that does not automatically mean you should turn off the entire filter. Maintain filtration and aeration unless the product instructions or an aquarium professional advise otherwise.

If the filter stops

Restore power or restart the filter promptly, following its instructions. Check that it is properly assembled, primed if required, and moving water normally. If the media has dried out or the filter was off for an extended period, test ammonia and nitrite closely and make water changes as needed. Don’t assume the cycle is unaffected or complete just because the filter is running again.