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.

Is there a leak in the suction line of the pump? It would pull in air
when the pump ran but leak water when the pump was off.