How do i find how much more water i need to fill an aquirium that is 3/4 full with dimensions 16in 8.5in 10.5in

The aquarium has dimensions of 16in and 8.5in and 10.5,
which sounds like a rectangular prism,
a rectangular prism has a volume of length * height * width
so the volume of this aquarium is

16 * 8.5 * 10.5

is 3/4 full now, that is, 3/4 of 16*8.5*10.5

how much much water it needs to get filled?
well, is 3/4 full, to get 4/4, all you need is 1/4