must the valv on the bottom of the pool be closed while filtering and only the skimmer open
No, you can filter from either the main drain or the skimmer. If you have all the suction going to the drain then you will be pulling most of the dirt and debris off of the bottom of the pool and if you have all the suction going to the skimmer then you will be pulling the debris off of the surface of the pool. Most people set the valve in the half way position so you are pulling from both the floor of the pool and the surface of the pool. I hope this helps, please let me know if you have any questions.
Thank You
Patrick
thank you yes we did this and the water is very clear now
Excellent thanks for posting back letting us know.
Patrick