typically the rule for sand/gravel coverage is 1lb per gallon tank for no plants. 2lb/gallon for thicker substrate layer./plants. IMO 1.5-2lbs/gallon looks the best, but I also like ~2 inches or so of substrate. 
Should be okay for 50-75lbs/75 gal, if you like a thinner layer. Can't wait untill you get the sand! Excited for you!
Should be okay for 50-75lbs/75 gal, if you like a thinner layer. Can't wait untill you get the sand! Excited for you!