Any sand in a tank should be kept open with a chopstick or similar, if you don't do this, you risk pockets of potentially hazardous anaerobic bacteria developing.
You DO NOT want aragonite sand. This is Calcareous, and will increase your hardness and pH. You want a neutral sand made from clean silica.
Pool filter sand is generally the best in my experience, the other types tend to pack down too hard too quickly.