The under gravel filter is a filtration system in which gravel is used as a filter. The gravel can be a good filtration material, especially if there is a substantial layer, more than 5 cm in height, of gravel. The gravel provides mechanical and biological filtration if the aquarium is well-cycled.
The working mechanism
The under gravel filters are usually made to be used with air pumps. Since air is lighter than water, it pushes the water up the filter pipe and back into the aquarium. In order for this to work properly, the air tubing should be placed down the pipe to create water flow. The use of an air pump is more effective on smaller tanks as opposed to large ones. The under gravel filter is sometimes used with a water pump (power head), which when correctly placed in the upper end of the pipe of the under gravel filter, a bigger water flow is created, thus, more effective filtration. A water pump is also needed when an under gravel filter is used in an outside pond. Since the pond is very big compared to the home aquarium, a strong power head is used to make current of water that goes through the gravel.
When you decide to use under gravel filter for your koi pond, an additional retaining wall should be underwater so the gravel keeps its shape and height in order to be a good filtration material.The setup is the same: first, you place down the under gravel filter grid, then you put the gravel on it. When everything is connected and the water pump is turned on, the water is drawn towards the gravel, sucked through the small holes of the under gravel filter grid and returned.
By cycling the water through gravel that has colonies of beneficial bacteria, you are accomplishing mechanical and biological filtration.