I went with regular sand,, I have been kicking myself for it too
I just set up a tank with river sand which looks ok with the stone scape i did but if i had enough black tone it would have been used. I probably only have 1/2 a 5litre bucket left. Black background is on but too big so it needs cutting down to size before taping it to the tank.
I've been using the vinyl that sign businesses use to letter vehicles. I take a spray bottle filled with water and a couple of drops of dish soap and spray it on the back of the tank. This gives an opportunity to slide it around. Peel the backing off. Set it on the back of the tank, adjust position and then squeegee the solution out starting from the center. This will work out any air bubbles as well. To remove lightly score the vinyl into strips with a pen knife and peel off. If it's difficult use a hair dryer or heat gun on low setting.
I often wondered how having the sides black would look. Only one side of my tank can be partially seen, it's in an alcove between the chimney and a wall. When I look directly into it, it looks like the side are mirrored. I supposed I would lose this effect???