Thanks guys, I just moved house 2 weeks ago and had to empty the tank of everything. I decided to put no substrate in to keep good quality after the move. and I didn't really have the time to properly decorate the tank, I have some more pieces of rock and ornaments etc so I guess that's always good.
another reason Is I don't want my fish to be unable to find food in all the space. I do have one of those "looks like a stump" ornament with holes in it. I was thinking of dropping food into it so only the bichirs can fit through the holes...