When you think of filtration the first thing that comes to mind is some kind of device that strains fine particles out of the water to make the water clear. Many types of filtration incorporate biological filtration (bacterial breaking down of ammonia and nitrite into nitrate), chemical, and particulate. While a device is commonly used in aquariums to filter the water, it is not required to have a healthy ecosystem. The only filtration required to maintain a healthy ecosystem is biological.
The fish we keep in our aquariums come from many different biotopes. Some fish like stagnant pools of water, while others are found in fast flowing rivers. When deciding which fish to keep, you should do the research to find out where the fish are found in the wild, and try and duplicate similar conditions in the aquarium.
Some current, in most cases, should be provided in the aquarium, even for fish that like stagnant water, as aquarium lights will transfer heat. If the aquarium is totally stagnant, the top few inches can easily become 10º F (6º C) warmer than bottom few inches. In nature, when the wind blows across even small pools of water current is created.
For most fish that we keep in aquariums, a turnover rate of 4 times an hour is adequate, but in some cases more or less would be preferred. In aquariums with river fish that like current, a turnover rate of 10 times an hour or more may be best. Many anabantoids would prefer a calm surface in which to build a bubble nest.
The bacteria that transform ammonia/ammonium/nitrite into nitrate colonize surface area in an aquarium ecosystem that receives constant contact with the oxygen rich water column. All surface in a filtration system is also colonized by beneficial bacteria. Many aquarium filter manufacturers incorporate some form of dense material in the filter design that allows it to be heavily colonized by nitrifying bacteria.
Filtration systems that incorporate biological filtration, can have an adverse affect on the aquarium after an extended power outage, or a thorough cleaning. Since materials used in filters for biological filtration are in high flow areas they become heavily colonized by nitrifying bacteria in densities that are in higher concentrations than what is in the aquarium.