Wow my own tread got away from me a bit on this one!
I'm going to experiment with a few ideas when I have some spare money.
I'm going to try epoxy and sand. Starboard if I can find it (or something similar) cheap locally. The tiles. And painting the outside bottom.
I have some spare 3gals lying around to be my test subjects :)
The plan is to put the fake sandbed in place. Add some dry rock to the tank and fill it with fresh water an a small pump and leave it in sunlight. After a few days there should be some kind of algae growth. This will give me and idea of how it will look in a marine tank with some colour on it. It's not a perfect plan with perfect results, but it will serve it's purpose.
Once coralline starts to grow, the plan is to let nature take it's course. There's not much I can do once that happens. The idea behind the fake sandbed was to put in in my fowlr when I upgrade, so I may just add a few sea urchins to keep the coralline at bay.