Water level down each day unless I run the fountain and other newbie ??

I put a small pond in my front yard. It has some very small gold fish
that were not gold when I bought them, but are now turing gold. In any
case, the pond is a 60 gallon pond and each day that I turn the
fountain off, I wake up to find the water down by four to five inches.
So I drained the pond and checked for cracks and found none. It still
went down over each night, it was becoming a problem to put water in
it each morning. Then I left the filter/fountain on at night, I was
turning it off. With the fountain on, the water level is fine in the
morning, now two mornings in a row.
OK, I don't know much about ponds, but I'm willing to learn. What
could take so much water from my pond at night? If something is
drinking it, they have one huge bladder. So what is happening to it?

One other thing is that I am using well water for my pond, free of
sulfur and rust. I have not tested it yet, but the fish have been in
it a week and most have survived. The few that did not showed signes
of being gutted like someone would do to clean a fish, cut from one
end to the other on the underside. No other damage to the corpse.
Thanks for any help.. We love the pond and want to learn enough to
feel good about putting a much larger pond in the back yard. I live in
South Florida, east coast, Jensen Beach. Joe