A ball valve and non-return valve are there to shut off water flow or water returning to your Koi pond and bleeding the system, or if the pump is taking a long time to start pumping again. If your pump is installed higher than water level, when you switch of the pump to clean the filter all the water runs back to the pond leaving the piping empty when the pump is restarted. When your pump is installed lower than the water level, water will flow out of your pump when opened, thus ball valves or non-return valves should be used in both situations.
The difference between ball and non-return valves is the following:
Simply, a non-return valve does not have to be closed as it closes itself, but non-return valves get blocked with debris and need to be stripped and cleaned. Ball valves don’t get blocked with leaves and other debris.
The down side of a ball valve is that you have to remember to close it when shutting down pump or pumps and to re-open before starting pumps.
My personal conclusion is to use ball valves and remember to shut and open them, easy peasy…