Waterfall pump with broken off ground pin in 3 prong plug unsafe to use?
I will assume that you live in a region where fake ground pins are common. (In my own experience I've found that "fake" ground pins are uncommon or even abnormal in the USA, but I have seen them in the UK.)

To check if it really is a "fake" ground pin, you could:


Confirm if the broken pin was actually metal, not plastic or similar. If plastic, it was obviously not a conducting element. If metal, its still uncertain.
Cut off the broken plug. If there are 3 wires then for normal residential wiring, it is grounded. The ground wire would typically be a standard color (check what's normal where you live). Since it is broken, this is a harmless thing to do.
If for some reason you don't want to cut off the broken plug, then you could use the continuity (resistance) setting on a multimeter to check if there is a current path between the stub of the ground pin and a metal part of the pump. While this can confirm that it was a real ground, it cannot safely disprove that it was fake. If the pump has only a plastic casing then this won't work either.


Once that is settled, you will know what kind of plug to replace it with. In my experience (again - USA and UK) a matching plug from a hardware store will be fine. Plugs are designed for a certain level of current &amp; voltage, and their shape determines which receptacles they can safely be used in.