How long can filters remain off during maintenance?

A filter should never be offline for more than 4-6hours during maintenance. The beneficial bacteria that live on the media will stay alive in a damp environment, but will begin to die after that vital period. If the media in a filter is dehydrated for longer than 8 hours, a significant percentage of the beneficial bacteria will be dead. Even worse, this causes spikes of deadly ammonia, and fish can die overnight from this. I try to keep all maintenance for any tank under four hours.

Bacteria form biofilms on filter media that trap moisture and oxygen. They can survive periods of shutdowns when kept fully submerged. Never allow components to dry out when cleaning and keep media immersed in tank water. Temperature is also important: it can accelerate die-off when kept in air (and especially cooler water) at less than 68 degrees.

Get everything set up before you shut off your filter. You'll need tools, media buckets, and other necessary items. Work swiftly! The less time you spend offline, the better. Should you find yourself lagging, keep your media in aerated tank water; battery-powered air stones can be a lifesaver in this situation. Keep a close eye on your fish once you bring everything back online.

If you experience a longer power outage, use uninterruptible power supplies to keep the most critical filters running. If the power is out for a while, remove the media from your filters and transfer it into a container with tank water and an air stone. Place one or more aquarium heaters in there to maintain the temperature. Your bacterial colonies should be able to survive like this for several days.

Read the full article: Aquarium Filter Maintenance: Essential Care Guide