Two different undergravel filters, which operate according to the flow-through principle or the suction principle.
undergravel filter flow-through principle by EHEIM:
The cleaned water is passed through the bottom of the soil, flooding the dirt particles and bringing them into the suction area of the filter.
undergravel filter suction principle by EHEIM:
The suction principle operates in the opposite direction and acts as an additional surface filter.
It is especially suitable for seawater aquariums and unplanted pools.