For biological filtration you need organic waste, bacteria and fungi. And water. Nothing else.
Sand or rock provides surface area for biological growth.
If sand or stone is used as a filter, it is a mechanical filter, but usually a mix of both due to the presence of organic waste.
A skimmer, formerly called a protein skimmer, does not in principle use biological life as a filtration medium, although biological degradation does of course take place, as is the case everywhere where organic substances are available. Skimming the water in an aquarium will be difficult without using mechanical means.
The use of a skimmer in a small aquarium can be very dramatic due to the very selective removal of organic substances and the failure to remove inorganic nitrogen compounds.
Biological filtration is based on complete decomposition and the availability of all building materials for rapid new growth, which should ensure pure water. This requires all building materials and not just those that are not removed by a skimmer. To keep the balance and clear water new growth must be harvested.
The use of a skimmer has a major influence on the nitrogen reserve and the rate at which nitrogen can be used up.
ref: The protein skimmer
Sand or rock provides surface area for biological growth.
If sand or stone is used as a filter, it is a mechanical filter, but usually a mix of both due to the presence of organic waste.
A skimmer, formerly called a protein skimmer, does not in principle use biological life as a filtration medium, although biological degradation does of course take place, as is the case everywhere where organic substances are available. Skimming the water in an aquarium will be difficult without using mechanical means.
The use of a skimmer in a small aquarium can be very dramatic due to the very selective removal of organic substances and the failure to remove inorganic nitrogen compounds.
Biological filtration is based on complete decomposition and the availability of all building materials for rapid new growth, which should ensure pure water. This requires all building materials and not just those that are not removed by a skimmer. To keep the balance and clear water new growth must be harvested.
The use of a skimmer has a major influence on the nitrogen reserve and the rate at which nitrogen can be used up.
ref: The protein skimmer