This might be relevant to your question... Type of air bubbler
An air blower, also called a bubbler or an air pump, introduces air into the spa water through a series of inlets. Many spas use the blower to provide massages. Spas can use it to mix air with water going through the jets. The air blower makes the bubbles. Air blowers for spas and hot tubs are essential to the experience. If there isn’t an air blower installed for the spa jets, then you might find a PVC pipe sticking up out of the ground with an open slit cut into the top of it, providing a way for air to enter into the jet system.
