I am interested in building a 6-8 inch high ledge of gravel in my tank that steeps off steeply to the bottom of my tank and will be around 10 inch long (out of 48) and 13 inch wide (out of 13) in my 55g. I am also interested in putting a small cave in the gravel side of the cliff (possibly) where fish can go. What do you think of this? How would I make the slope? Thanks!