That is a corner filter. It looks like you have filter wool in it at the moment(or is it a sponge), and is that small gravel in the bottom of it? Is there one or two? How large is your tank?
When cleaning filters never change all the media at once!!!! Change 1/2 media once every one to two months. Depending on how full tank is, how much filtration, and how diligent at sucking up old food you are.
My corner filters use three types of media. Activated carbon on the bottom, filter wool in the middle, and ceramic noodles on top(absolutely the best invention ever).
One month replace the filter wool. And rinse the ceramic noodles in TANK WATER.
Next month replace the carbon. And rinse the ceramic noodles in TANK WATER. New carbon needs to be left in TANK WATER for a few minutes, to let all the trapped oxygen escape, and rinse it of dust.
As long as one of the medias isn't cleaned it will retain enough bacteria to continue to nitrogen cycle in your tank.
Just a few tips: Keep your established filter media well away from fresh water while doing this. The last thing you want is to accidentally knock it into a sink, and destroy all your hard working bacteria.
Hope this answers all your questions.