What about coating the pond in something like pond coat, and while its still tacky, placing tile on top of it? This would give you the waterproof membrane, plus the look of the tiles, and you wouldn't have to worry about grouting them, or worry about the wood flexing and breaking their bond with the mortar.
Not that this would be cheap, but it might be another option if you have the funds and really want a tiled tank.
Not that this would be cheap, but it might be another option if you have the funds and really want a tiled tank.
