When power is lost for an extended period, the bacteria on these materials can use up the oxygen in the surrounding water quickly and then start to die. When the filter is restarted, a significant portion of the ecosystems beneficial bacteria have been lost, thus resulting in a significant ammonia and/or nitrite spike a few days later that can cause fish mortality.
When cleaning biological media, significant amounts of beneficial bacteria can be lost. Many hobbyist suggest using aquarium water to clean biological media, but even with aquarium water beneficial bacteria can be lost. For most aquarium systems, it is best if the biological filtration occurs in the aquarium and not the filter.
In fish hatcheries it is common to see trickle filtration systems with towers filled with bio-media. The bio-media is used to support high fish stocking densities in fish hatcheries, where tanks are kept bare for easy maintenance and have very little surface area.
The most common forms of chemical filtration is the use of activated carbon and zeolite. Chemical filtration is not necessary for maintaining a healthy ecosystem. Chemical filtration has its purpose in the hobby, but it is not recommended for regular filtration. Chemical filtration does not remove nitrate, and will not extend the time between required water changes.
Many filtration systems include some activated carbon. Activated carbon is effective at removing most medications and tannins that make the water yellow or tea color. It can also remove beneficial trace elements, that are used by plants.
Zeolite is a white mineral marketed for freshwater aquariums to pull ammonia out of solution. Zeolite does not work in saltwater. Zeolite can be recharged by soaking it overnight in saltwater. The use of zeolite is not necessary for maintaining a healthy aquarium. Once zeolite has been added to the filtration system it will eventually become colonized by nitrifying bacteria and become biological filtration media.
Another form a chemical filtration that is sometimes used to control algae growth are phosphate removers. Phosphate removers have proven to be effective in cases where high phosphate levels fuel undesirable algae growth. Keep in mind when using these products if you are trying to keep live plants, plants need a trace amount of phosphate for proper growth. To little phosphate in the system can affect plant growth.