Here are the basics for how to check for a leak in your pond
First, check the float valve to see if it's running constantly (faster than a drip). If not, then you don't have a leak, and you might want to check elsewhere, like your irrigation pipes. If so, continue with the next step.
Determine where the water level is at the moment (because turning off the waterfall and/or stream will overfill the pond temporarily). Either eyeball and remember, or put a piece of tape somewhere to note it.
Turn off the water supply to the feature and unplug the pump for 24 hours, and note how low the water in the pond drops below your original mark at the end of this time period.
If it does drop, then continue to monitor it in 12-24 hour increments to see how far it drops, until it stops dropping, which will tell us at what level the pond has a leak.
If the water doesn't drop, plug the pump back in, leaving the water supply off. The pond will drop initially, until the waterfall and/or stream are back up and running fully. Once that's happened, note how low the water in the pond drops over the next 2-4 hours. This second step should NOT be done without someone checking on things every couple of hours, because if it's a serious leak, this could drain the pond unexpectedly.
