how to figure out net volume?
Are there any tricks to figuring out my net water volume without knowing how many lbs of live rock, and how much sand is in my tank? I have a 300g tank about a 1/3 to 1/2 full of rock, and 23 inches of sand depth. Is there a calculator out there to help?

Mmmmmm.... Let me recall my Physics 12. :lol:
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=VtV1 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 
You buy another 300g tank. Set it up lower down but beside your display tank. Drain display into new tank. Measure the inside dimensions & use L x W x H/231. That will give you the exact net water volume.:wink:
No complicated university level math involved. 
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To simplify, I'll guess the rock takes 50g of space. There ya go! 
Just take your total volume and multiply by 80% (0.8) to account for rock. That's close enough for Zeo, it's even in the guide.
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