Move your fish to a separate tub filled with their old water.
Turn off the system and any additional devices like an auto-fill.
Refill the pond to the normal level.
Allow the pond water to drain.
When the water stops, check the water line for the source of the leak.
If the water level doesn’t drop significantly, the leak isn’t in the pond basin and you’ll need to search elsewhere.
The Milk or Dye Method
Use a gallon of milk or non-toxic dye to locate a pond leak. Pour the colored liquid from the top of the feature and watch where it goes. If there’s a hole, the milk or dye should flow into it and out of your pond.