The recent development in saltwater tanks in recent years has been the delopment of the Deep Sand Bed (DSB). Comprised of usually at least four inches of sand on the bottom of the tank, the sand bed will have anaerobic zones where oxygen does not reach. This allows a place for nitrate-eating bacteria to grow.
The big quesiton. once depth was determined, was the make-up of the sand in the sand bed. Since what is needed is bacteria growth, surface area is one of the more important factors in sand. Per cubic foot, the smaller the grains of sand, the more surface area. As a result, frequently calls for "sugar-like consistency" is heard when talking about sand and its crystal size.
While coral tanks may require more specific sand in terms of chemical make-up, for a fish-only or octopus tank, most any sterile, clean sand will do. After a lot of research (mainly a lot of reading and a lot of driving around to home improvement and harware stores with plastic baggies and vinegar), I came to two conclusions:
1. Silica or argonate sand does not matter too much for a fish-only or octopus tank.
2. It is impossible to find Southdown (or Old Castle) sand in the Chicago area.
I will not go into too much depth as to what that second statement means, but I will say that, as a result of those two things, I ended up buying the 250lbs. of plain-old silica-based sand that you see above.
For my 75gallon tank, 200lbs of the sand would create a sandbed of about 5 inches deep. I bought another bag for use in the sump or in another tank. At less than $3 for 50lbs, I could afford the extra bag. This can be compared against the going rate of about $5-6 a bag for Southdown, and the off-the-charts price for "saltwater tank sand".
I bought the sand above after visiting a lot of hardware stores and finding the sand with the best consistency and looks. The sand is fairly white, unlike a lot of other silica sand, which can be rather brown at times.