It is suggested by some to have at least 4 to 6 inches for good nitrification properties. There are a lot of contraversial views from the experts. Some say sand, some say no sand.
I have 3 in. of sand and somewhere around 75 lbs of LR
in a 55Gal
It is hobby and you can do it however you like...Who knows, maybe you will come upon something to contribute to the way it is done....