The Formula for convert API gravity to pounds per gallon is: Pounds per Gallon = (141.5/(API Gavity+131.5))*8.3378 <--[which is weight of fresh water] API of 10 is same as weight of water. API >10 will float
The actual weight of the aquarium itself would depend on the materials used. The rocks in the bottom would add some weight as well. If you want to know how much the water weighs. The mass (weight) of fresh water is about 8.35 pounds per gallon, so that is 459.25 pounds for the water.
The density of seawater averages about 1.025 grams per milliliter. With conversion factors of about 3,785 milliliters per gallon and 453.6 grams per pound, this means that a gallon of seawater comes to roughly 3,879 grams or just under 8.5 pounds in weight.