how to figure out net volume?

[crimper]

Mmmmmm.... Let me recall my Physics 12.

If you don't have the total weight or total volume of your sand and rocks, it's pretty hard to to get the exact volume of your water. However you can use the Buoyant Force Formula (Archimedes Principle) to get the closest volume.

Buoyant Force Formula
Fb=gρV=ρghA
Where ρ is the density of the fluid (saltwater=1029 kg/m3)
g is the gravity,
V is volume of the immersed part of the body in the fluid.
h is the height of immersed part and
A is the area.

Solve for Fb (Buoyant Force) first
Fb=ρghA

Then Solve for V (Volume of the immersed objects e.g. sand+rocks)

V1=[Fb/ρg]

Vt=V1+V2
Where Vt= Total Volume = 300 Gallons = 1135.62 Liters
V1=Total Volume of Objects (Sand+Rocks)
V2= Total Volume of Water

V2=Vt-V1

Please note the units that you're using, it is easier to convert everything in metric (Liters) then convert the final answer to Gallons.

Good Luck