Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
Don't know what kind of sand you're thinking of, but I'd suggest you seriously consider quartz-based silica pool filter sand over any other type of sand, if you can get it (from a pool supply store).
Do yourself a favor and avoid any 'play sands' available at the big box stores, and I'll be glad to tell you why if you want to know the details.
Other than that, any specialty sand sold by LFS's will usually be quite expensive, whereas PFS can be had for $10. to $15. for a 50 lb. bag.
And that's about all you'd need to do a suitable job in your 75 gal tank, by sloping it slightly from say 1" to 1.5" in front, to no more than 3" at the back. Any deeper than that depth of sand, and you significantly increase the risk of anaerobic gas pockets developing - and plants can be rooted well in 2.0" to 3.0" of sand depth.
Hope this helps.