Can you please supply a picture? It will help some. Also, you say the water level doesn't decline if the waterfall isn't on. But it does when it's on. That most certainly indicates you have an issue with water loss related to that.

1. Check all lines running to/from the waterfall. How is it pumped up? Through an underground line, or above ground?

2. You said you checked the liner, but did you check under the liner? Like peel it back from the ground? It sounds as though you have a leak, you need to look everywhere for it.

3. A cracked filter housing could certainly result in loss of water. Have you looked around/near the filter to see if you're losing water from around it?

4. What in the world is a $700 quote for? Again, a picture will be worth a $1000.

More after you respond.

There may be some experienced 'ponders' in your area that would look at your situation and give you some help.

You know the pond itself isn't leaking, Move the hose from the waterfall to another area and secure it, leaving it over night and see if you still have water loss. If none then you know the fault is in the waterfall area.

As others said, the leak could be anywhere in the pumping tubing once it is out of the pond and of course also includes the waterfall until it gets back into the pond. The easy way to loose lots of water fast is to have a plumbing line pop off somewhere. If there is an external tank filter of some kind that is outside the pond that is also a likely spot. See if you can determine the plumbing layout. You may not have a separate filter. And yes, $700, huh? BTW, you can always leave the waterfall off for awhile. You could always have a pump running right in the pond circulating water by pumping it around under water. This might be more complicated if you have an external pump.

Turn off the pump and let sit for a whole day, if the water level doesn't move you will know the liner is holding. If it stays, turn on the pump after a day. I would say check the complete water line from pump to waterfall, some where the ground has to be soaked.

I was wondering if you tried running the water fall without the rocks...it could simply be that your water is wicking through a small gap in the rock/s and this could be easily and less costly to fix.

Just thought I'd throw this out there if you haven't thought of it already.

Kathie

We no nothing of your plumbing situation yet, but the way I attempt to distinguish a waterfall failure from a big plumbing break is to connect a diverter plumbing pipe from just before the waterfall that goes back to the pond. If you are still loosing water, then you have a plumbing failure.

Typically, there is a pre-filter type thing on the pump which is submerged them the pump has a tube that leads to the filter. The filter is a big box type thing which is usually a stand alone unit and can include a uv light which helps to keep the water clean.

My guess would be that the leak is it's somewhere in the waterline leading to your waterfall. One fix would be to get new tubing from the pump to the waterfall and see if that helps. The tubing on my pond was all buried so i got a stand aloneI submersible pump and put it in the deepest part of the pond, then tubing leading out of the pond into the filter, which is sitting on top of the waterfall. The water footers through then dumps onto the waterfall and back into the pond.