If your pump is a rotary vane model I suggest you use at least a check valve on the diffuser. The make all plastic check valves to the do not corrode and stick as much as a metal check valve..

Some bubblers have built-in check valves and if you can use one of those that would be very adequate for your application. Then a check valve would not be needed at the pump. Some pondowners want a double check valve, then they also put one at the pump.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management