I read that when starting a SW aquarium I should put the rock on the bottom first then distribute the sand around it. There were two reasons for this, one: rocks placed on sand could cause the dangerous gas pockets in the sand. The second reason is that potentially alot of pressure could be put on a couple grains of sand and put unnecessary pressure on the bottom of the tank. Are these bogus reasons? Or should I follow this rule and put down the rock first? Also, for the sand whats a good amount for a 75g. I would like around 3 inches of live sand.