Swimming pools blog

I have a Pac Fab Triton TR40. I don’t know how old it is so I
thought I’d change the sand.
I unscrewed the glass dome from the top. Under this, just inside the
hole is a white plastic round thing with holes in it (presumably to
distribute the water over the sand). I can’t seem to remove it
without undue force. Can anyone tell me if it unscrews, pulls off or
if there is something else I need to do first.
Thanks
Nick.