If you see the black sand from Petstuff101, it's very very fine gravel, of the 'midnight' variety, not 'sand' strictly. The normal midnight fine gravel looks really good (I've got it) but I'm not sure on the suitability of even fine gravel for fish with barbs like the khuli loaches. They would try to burrow as it would be light but I suspect it could be much rougher on them than sand. Since the theme is what counts most in this case, maybe you could think about getting another eye catching fish and still get the black gravel?