When the pump has been running in filter mode for awhile and I turn the motor off, there is a big rush of water and it gushes back out through the skimmer. Strong enough to pop the lid off. I've been running the filter for about a week now, and it just started doing it today. The filter normally runs at about 10psi and I've been backwashing it when it gets to 20psi or above.
What should I be looking for?