Live Sand

In most cases, it's completely up to you (do you have a skimmer?). You don't really even have to have any substrate (reffered to as bare bottom). You could go with a 1-2 inch sand bed, wich is mostly for looks. You could also go with a deep sand bed, over 3 inches deep, wich boasts denitrification capabilitys (I recomend it if you aren't running a skimmer).

A rough calculation for how much substrate you need is 10 pounds per square foot, per inch of desired depth.
 
Also you don't have to or need to buy live sand . You can use any arogonite based sand and it will turn to live sand in short order. A lot of the critters in the live sand will move off the live rock. You can also get a cup of sand from another reefer or off line.
hth
Max
 
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