I have old pool returns which are PVC tubes that are probably 1 or 1.5 inch holes. They come out at an angle from the plaster wall as to circulate the water I assume.
I wanted to install a PVC water fountain to the returns to areate and cool the water at night.
However, most fountains require a 1.5 threaded coupling at the pool return, which I do not have.
You could add a female fitting to the pipe coming out of the pool, so if the pipe coming out of the pool was 1" you could go with a 1" slip x 1.5" female fitting to glue onto the pipe or if the pipe was 1.5" then go with a 1.5" slip x 1.5" female. The slip portion of the fitting would slip onto your pipe and the water fountain would then screw into the 1.5" female.