Several years ago I obtained a pond when I purchased our present house.
It had what I considered significant leaks. After several post to
this group asking about ideas to find leaks and many hours over the
period, I believe I have finally found the source of my main leak.

To find this leak I have stop the pond pump and watched the level of the
three pools, to determine which pool had the leak and the approximate
level of the leak. I found that the best caulking for leaks was roofing
asphalt.

Over the last few years I realized that the most significant drops in
the pond level were random. My first clue to the source of the random
leak was found when our cat was making strange noises at the family room
door to the deck where the pond is located. When we turn on the deck
light, there was a family of raccoons "talking" to the cat. When they
saw us they immediately went into the begging position, sitting on their
rumps with the palms on the front legs extended.

Then I realized there were wet paw prints on the deck when the level
drop, I realized the the source of the "leak" was the pool party that
the raccoons having on "nice" evening. (I have not determined what a
nice evening is to a raccoon)

Since this female raccoon has now raised several families in our yard, I
suspect that I will have to learn to live with the leak.