When I had my softy I used play sand. If you use a preformed pond liner I think it would be fine to use play sand or any other sand you like. The only problem I see is with filtration. Since you must keep the water very clean, your going to need a good sized pond filter. Most of these sit on the bottom of the pond which would cause it to just suck up sand, wrecking the motor...
Chris
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Hey whats up, I'm Chris and I currently have 0.1 Midland Painted, 1.0 Western Painted, 1.0 Bearded Dragon