I have a backyard pond, with common goldfish. It is well maintained, with a small knee high fence around it, which is completely covered by a net canopy staked to the ground. It is currently winterized, with one small trickle fountain filter in the center, and a pond de-icer which was turned on as the nights here are now getting below 32 degrees. The de-icer is also staked to a deep submerged potted plant, so it will not run into the sides of the liner. The pond liner is a hard molded liner.

This is not a recent problem, but a mysterious one that makes no sense, its happened all summer, and now, even that it is winterized and covered with netting. Here's the problem.

Pond runs fine, for weeks, water level stays the same, no equipment leaks, and except for an occasional bucket of water to be added for evaporation, its fine. Three weeks will go by, and then suddenly we wake up, and 1/3 to half of the water is mysteriously gone. We check it, and there's nothing amiss, no equipment leaks, and the fountain is just on a trickle so there is no splash out from that. We refill it, and it holds water just fine, go a week, two weeks, water level is fine, then bam...... wake up, a massive amount of water is gone.

I have a hard time believing the molded liner is leaking, as it holds water, for quite some time, in all temperatures, then just suddenly, pouf!

Anyone have any ideas what's causing this? Has anyone had a similar problem? I'm at my wits end!