I have a cast concrete fountain sitting in my informal pond.
I did it all wrong the first time. The second time .....
I leveled and tamped the sand where the fountain would sit. Then I put saltillo tile where the fountain would sit. Then I put an old bathrug over the tile. Then in went the liner. I used scrap pieces of liner to sit more saltillo tile on top of the saltillo tile under the liner. Finally, I sat concrete block on top of the saltillo clay tile. The concrete block openings are to the pond so the fish can swim through them and hide out inside the block.
Lastly, the fountain came with it's own pump and is intended to be a self contained system. I didn't much like that method, so when we redid it, I branched off the main pump to feed the fountain. Water from the fountain basin overflows back to the pond. Creates a nice effect, and helps with filtration.
My fountain is a cast concrete (sealed) fountain. It takes three men and a gimp to get it in place. Don't need to worry about it blowing over in a storm. Making sure the surface is level, and will not settle after it has a heavy fountain full of water is kind of critical.