This is a satellite filter cage designed for use with the Oase Aquamax Eco Premium filter pumps. The pump has a dual inlet and requires a satellite like this one or a skimmer to be connected to stop leaves and large debris from being drawn into the pump and blocking it from the second inlet.
Designed for the Oase Aquamax but can be used with any brand. The satellite cage is installed on the end of the hose that connects to the pump. It can also be used with external pond pumps that are dry mounted to protect it from leaves and large items.