If done correctly they are none and in fact the old time tank had slate bottoms.My first tank had a slate bottom.The hardest part of using slate tile will be getting a proper fit around the perimetet of the tank do to the silicone holding the glass
now if you have acess to a wet saw have at just cut and fit the tiles.I had a wet saw but it got lost during a move -Anne
now if you have acess to a wet saw have at just cut and fit the tiles.I had a wet saw but it got lost during a move -Anne