I have a 55 gal. I went with 2 lbs per gallon. 40 lbs in a 125 gal tank is not enough. You will have a little tiny pile of rock in the tank. If your looking to save some money get 150 lbs of base rock and another 50 lbs of live rock and let it seed the base rock. It will take about 2 months for the base rock to seed, but it will save a lot of cash. I did 80 lbs of base in mine and 30 lbs of live rock, putting some in my sump and after two months, the base rock was looking very good. I also used Purple Up as an additive, it seems to work. I have coraline alge growing everywhere.