[Aqua Pro Builder]

For indoor pool and flower horn, I think you don't need 4" water height. If you choose 18 to 24" the wood to support for the water pressure will be much less.

DON'T use tarp, use pond liner. You need the cushy thickness of the pond liner. Make sure you rounded all the inside corners of the "tub" with some cushy stuff-it will extend the life of liner. Make it into a bowl instead of a box.

Make sure you also cosider fencing your pond so that your fish will not jump out and no children can jump in. flower horn can jump quit far.

If budget allows in the future, go for the Japanese garden building style. It's very mobile-almost everything are reusable too. I always move my ponds around from time to time.