Filters should be run all day every day (24/7).
They develop colonies of beneficial bacteria over a period of a month or so and these bacteria help keep the water clean. If the filter is not allowed to run continuously the good bacteria in it will die from lack of oxygen. So the filter has to be run all the time.
If there is a power failure the bacteria will be ok for a few hours but will start to die if the power failure lasts more than about 3 hours. If there is a really long power failure you can keep the bacteria alive by putting the outlet hose into a bucket and letting the bucket fill up with water from the filter. Then put the outlet hose back in the tank and pour the bucket of filtered water back into the tank. Do this about once an hour during the power failure, and the new water flowing into the filter will provide sufficient oxygen for the bacteria in it.
When the power comes back on then just let it run like normal
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When established, power filters should be cleaned at least once a month, however do not clean a new filter for the first 6-8 weeks otherwise you can interrupt the bacteria that are trying to grow in it.
To clean a power filter.
Turn off and unplug the filter and drain the water out onto the lawn/ garden.
Open the filter and remove the filter materials and squeeze them out in a bucket of tank water. When they are reasonably clean, squeeze them out in another bucket of tank water. Then put the filter materials in the tank.
If you have carbon (black granulated stuff) in the filter, then throw it away. It absorbs chemicals like heavy metals, medications and plant fertilisers and is not normally necessary. You can put another sponge in the filter where the carbon is.
If you have an ammonia absorbing granule (looks like white gravel or white powder in a bag), then throw it away and don't replace it. Ammonia absorbing granules can interfere with the filter bacteria and stop them developing properly.
If you have ceramic or plastic beads in the filter, rinse them in a couple of buckets of tank water before putting them in the tank while you clean the rest of the filter.
The filter case and motor unit can be rinsed under tap water to remove any gunk. In the motor is an impellor, which consists of a black cylindrical magnet that has 3 or 4 plastic blades on one end. The impellor usually has a steel shaft running through the middle of it. The steel shaft usually has a rubber bit on each end to stop it coming out of the impellor. Carefully remove the rubber grommets on the ends and take the steel shaft out. There might be plastic washers under the rubber grommets so look for them. Clean the steel shaft, rubber grommets and impellor under tap water, then reassemble them.
Put the impellor back in the motor housing and put the filter materials back in the filter case. Put the filter back together and connect the hoses up. Put the outlet hose into a bucket and let the tank water drain into the filter to fill it up and start flowing into the bucket. Then put outlet hose in the tank and plug filter into a power socket and turn it on.