Just to put a stop the live rock to gallons BS everybody is spewing...... there is NO set number/ratio/equation until you pick out what rock you want! In a 75g tank for example you may want 150lbs of tampa bay saltwater rock and that wont look like much, but 150 pounds of tonga may look way over crowded and 150 lbs of BRS pukani or marco rocks wouldn't even come close to fitting in the tank if you just dumped it in and didn't add sand or water, you would like need 3 75g tanks for that ammount! Wait till you are ready for rocks, pick a few diffrent kinds then specifically ask how they look in seperate threads so you can stop the argueing between folks, something like "If I get the type from this company what can I expect?" you may also want to be decided on what kind of formations you want in your tank, islands, minimal, rock wall, and so on before you ask so you can get specific and ask "I want this and this in this size tank this rocks in this formation of this type of rock; what would you suggest for poundage?" and you will be getting some pretty dang good and specific advice!