I believe your instinct was right. Get the tank planted and look at it for a couple of weeks and make changes needed until you are satisfied with it. Go back and read any care and setup info and compare it with yours. With mantellas it should be simple. The little buggers are great at hiding. I would make the substrate medium pebbles and in a color which contrasts with the color of the mantella. The less handling after you get it all set up the better. Good luck.