I wouldn't go with HD sand, as it may contain chemicals, anti-fungal stuff, and lots of nasty things. Check with your LFS and see if you can get a bag of filter sand. 0 point gravel is what it's technically called I think, and it works great. I've used it in fresh and salt, with a lot of success. I usually pay $15 for 50lbs of it, so a little money goes a long way.