You probably don't have 30g of water in there. Do you have 20g? 25g? 28? If you can estimate how much water is in your tank (use the tank capacity calculator on the home page here and measure from the sand to the water line and the length and width) then you can divide that by 2 to figure out how many cups you need in the tank. We can't tell you how much because we don't know how many gallons of water you have in there. On top of that you're estimating salt amounts. You're just going to have to give it a good guess and see where you end up.
Join the NJ Reef Club
Today! Saving the world's reefs one living room at a time!
Atlantic City, NJ September 25-27, 2009