What is the best material for the floor of a garden foutain pool

Browse though what others have done. https://www.hometalk.com/categories/outdoor-living/ponds-water-features/ponds

Best and only material to use is something called "pond liner". Anything less and the water will escape.

First you need to lay down a rubber membrane. Check out these sites on Pinterest. Type DIY Koi Pond.

A friend used Rubber Roofing material for his. I believe the piece he used was about 10' wide. I think wider pieces are available but its been a long time since I've used any. Check a local roofing supplier!

Black, flexible pond liner sheeting -- sold by the sq ft at pool supply or big box stores --

Waterproof and Frost Proof Concrete. or Preformed fibreglass

I used an old jacuzzi, (just the fiberglass insert) and it worked great! The seats were built in plant shelves!

My friend bought a plastic formed pond liner at Home Depot and tiled it with flagstone. Beautiful!

Anything that hold water and is not toxic will work.

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I have seen some really nice ones using a 1 ft tall galvanized horse trough... drain plug... but fewer chances of leaks

Pond liner