Not all Home Depots carry Southdown sand. They are usually just the ones on the east coast. If they do have it, let us know! :-) As far as how much sand to put in the 55g tank, it is really up to you. There are calculators that can figure out how many pounds of sand you need to fill a certain depth after inputing the measurements of the tank. The depth is up to you depending if you want to go with a deep sand bed or just a normal bottom. Many peoples sand beds do not act as true DSBs as they were not developed properly.