1-2 lbs per gallon is what I've heard.
I have 150lbs in a 90g so that's roughly 1.667lbs/g.
I like it because I have an eel, wrasse, goby, and trigger.
My eel has plenty of depth to bury in. My wrasse can sleep, my goby can eat, and my niger...well I haven't really figured out what they do with the sand other than make a mess.