Not sure about the "brighter" part but, I thought you might be interested. I went to (Oh I hate to say it) RC and used their "sand calculator" here's the link -
Reef Central sand calc
Put in the length, width and the depth of sand desired and hit calculate. For your 30 x 18 x 5" depth it brought back a figure of 109 lbs. I used a cubic foot of sand weighing 70 lbs as the denominator. That's what CaribSea claims is the weight. I'll take their word for it. If that's accurate you'd need 3 boxes minimum or 4 to be sure.
Dick
