Question

what weight of water is required to fill a 100 gallon aquarium

Answer 1

The density of water is about 1,000.00 kg/m³
There are 264.172 cubic meters in a gallon
Weight is mass times gravity.
So,
$$
\text{mass }=100 \text{ gallons}\times \cfrac{264.172~m^3}{1\text{ gallon}}\times\cfrac{1,000.00~kg}{m^3}\\
\text{weight }_n=\text{mass }\times\text{ gravity}=\text{mass }\times 9.81 \cfrac{m}{s^2}
$$
This weight is in Newtons (i.e. \(\text{weight}_n\))

To convert to pounds (i.e. \(\text{weight}_p\)): 1 pound force = 4.44822162 newtons
So, in pounds, the weight of 100 gallons of water is,
$$
\text{weight}_p=\text{weight}_n\cfrac{1\text{ pound force}}{4.44822162 \text{ Newtons}}
$$

That's it!