thanks guys, looks like id need 2 15x15 pieces to do the back. the sides on the other hand id probabley do a 12x12 and then cut 3 inches off another 12x12 piece to shore up the top. the slate is easier to work with than the marble comp. i saw one done in black swirled marble, it looked great but not natural to me. on this particular tank i had to do a little work on the corners to allow for the tank silicone, it was easy with the slate to just chip off the corners and work them to get a snug fit. when i had it exactly how i desired, i ran a bead of silicone around the edges and a bit in the center to keep the gunk from getting under it.
im sure they were pissed at lowes as i had 2 open boxes of this comp layed out on the floor trying to get 2 pieces that matched best so i kinda took up an isle for awhile. i may do this with a 55 in the near future which will be 3 full 12x12s and a partial piece. this stuff was only like 1.87 per piece so its cheap to do and gives a different look if looking for an alternitive to a barebottom tank which some of us desire for certain tanks. i have plans for some slate caves also that i may incorperate. ill use this 20 as my experimental model.
mike