Alec

If your pond stops going down before you have to pull your fish, you
might add about 1/4" of water and then begin dropping drops of milk
around the edge. Where the milk goes through the side is the lowest
leak. There could be several. Continue around the pond.

Patch the leak and either fill the pond or go up as high as you would
like next to check. See if it goes down. If so and if it stops,
repeat the leak check with milk.

If your problem is the pump, you should notice 300+ litres of wetness
near it!

Good luck.

Jim