I have a 30-ish hex... its about 24" tall, so I think a 30 hex would be about as tall... maybe a little less... it has 40 lbs of rock, but its heck to get aquascaped... I want to dig in there as soon as my salt gets here for a big ole water change/scrub down of the rocks, and rearangement.
but, 1 lb per gallon in a hex works fine, just have to be a bit more inventive than with other tanks.
the 30 (29 gallon standard size) is perfect for horses also!!! 1-2 lbs of rock a gallon standard also.