Ringo, I wouldnt worry too much about how much it can hold. My tank is a 55 gallon and i have maybe about 40 pounds in it. Your 75 would probably hold quite a bit more than mine so i think as long as it looks full then thats enough.(but i wouldnt go over 75 pounds :) ) Someone please correct me if im wrong
Water is over 7 lbs. to the gallon and forces out pressure on all sides of the tank. All the rock's weight is sent to the bottom and not the sides. Any rock you add also displaces water, so you could also subtract 7 lbs. per gallon of rock from the total weight. If you have a standard 55 gallon made by a large company it will have a tempered bottom. This makes the glass extreamly strong, but brittle in that it will shatter rather than crack. I would think that if you had quality live rock like the dead coral heads you could fill the entire thing. Just the water in the tank is over 350 lbs. and most of that would be displaced if it were filled to the brim with rock (and I have never seen 350 lbs. of live rock in a 55 - )
THANKS for the info guys. My tank is tempered glass all around, I was just a little concerned about the bottom with sand weight(45lb)and live rock weight(45lbs). My tank just seems kind of bare the way it is, I have seen pictures of other tanks that are just full of rocks. Anyways THANK YOU GUYS for the info.
i have about 90 pounds of live sand and 120 pounds of live rock in my 55 gallon!! ull be fine with 45!! if you want anymore, im tryin to get rid of mine for 3-4 bux a pound..... i have one large peice that weighs about 37 pounds lol