A deep sand bed is a sand substrate coupled with bacteria and multicellular organisms working together as a powerful biological filter. Originally developed in the saltwater aquarium hobby, deep sand beds (DSBs) extended the biological filter from converting ammonia and ammonium to nitrate, to eliminating nitrate by denitrification - using the oxygen in nitrate, releasing harmless nitrogen or nitrous oxide. This was desirable because of saltwater organisms’ great sensitivity to nitrate. In freshwater, nitrates are much less toxic, requiring infrequent water changes to control levels, but the freshwater DSB as described in this article does more than eliminate nitrate.
What is the best sand to use as substrate
…Aquarium substrates are pH neutral and will not increase aquarium carbonate hardness. Unique grain size resists collection of detritus, fish waste or food within the substrate bed. available... in a variety of natural gravels and sands to enhance the look and function of freshwater aquariums.…
I plan on having a freshwater tank
Play sand is something I have always considered a no no in a freshwater aquarium. There is never a way to know how heavy the mineral content is with it, and the grains tend to be very fine, which creates mud on the bottom... thus organic breakdown that can cause problems for the animals. If sand is absolutely needed, silica sand is the best and safest option. You should be able to purchase it at any lfs that sells gravel and other specialized plant substrates, or at most reptile supply stores. If its not seen on the shelf, you can ask for it. Some lfs's don't stock it regularly because there is such a low demand for it in most areas, but it should be easily available to them through their distributor.
white sand in a freshwater tank