What kind of skimmer should be used with an 18 ft X 48 inch round above-ground swimming pool if the pump being used is a Meteor 21 sand filter system?

Firstly, the pump is not a sand filter system. The pump is the one with the motor attached and it pumps water not sand. The large round tank is the "sand" filter. You would have to find a skimmer made for vinyl pools with gaskets. It attaches to the wall of the pool and is seen "hanging" from the outside of the pool. The type of pool and equipment have no factor in choosing a skimmer.

another good way to go is the Dragonfly Super skimmer this is a floating pool skimmer and only requires a hose to connect it to suction