I used a little over 20lbs of the caribsea for my 115g tank. It was about a half inch deep, so 40 pounds would have given me an inch of substrate. My tank dimensions are 48"x18". So adjust the amount you put in your tank to only reach one inch in height. I think 15 bags is too much, but you can always use the extra to fill in should you end up siphoning some out as you vacuum (I did that a LOT..lol)
I just got rid of my sand because with 9 growing, hungry discus it was just too much work to keep the sand looking clean and to not suck it all up while I stood there carefully hovering the siphon tube over the sand...blech.
Much easier to just suck the poop and food bits right off the glass bottom. Done in 5 minutes! I do think the sand substrate looks awesome though and I'll add it back later on when my discus are adults and not eating so much and so often.