I found this online. Hope it helps.
An overflowing skimmer is caused by two problems. The first is that the water level in the sump is covering the outlet pipe on the skimmer or is at the same level as the outlet pipe. When the water in the sump is running at or above the outlet pipe on the skimmer you are putting back pressure in the main cylinder of the skimmer. The water has to exit and when the outlet pipe is underwater it is pushing against the weight of the water in the sump. When the skimmer is new you maybe able to get away with this for a little amount of time, but once the skimmer is fully broken in you will have overflow problems all the time. You must either lower the water level in your sump or raise your skimmer in the sump by placing it on a small stand or even some cut off pieces of PVC
pipe. You can easily cut a few pieces of PVC
for the base of the skimmer to sit on at the depth that you need to raise the outlet pipe out of the water.
The second reason for overflow is that something is preventing air from entering the skimmer pump through the venturi. When air is restricted it allows for more water to be pumped into the skimmer. More water in the chamber will cause the skimmer to overflow. Make sure you check the air venturi and air hose for any salt creep deposits. Also make sure the air hose is not bent preventing air flow into the pump. If the air hose is bent reroute the hose so that it will not bend.