The previous owner of my house, who also built my pool, told me that when he built the pool, he ran pipes to the spa but never hooked them up. That is the gray pipe in the picture, and I assume the other end is the gray piece next to the eyeball in the spa. There are several of these in the spa. I can't remember what he said this pipe was for, possibly something about aeration. Is that a thing in spas? Currently the spa works just fine when I run it, with jets coming from the sides and the floor jets that can be seen in the picture. He called the floor jets "bubblers", is that a correct term?
