When I loaded the first half of my 50 gal package, I had the tank ~2/3 full and then topped off from there. With the sand and 1st half of the rock, I don't think it took up more than 1/4 of the volume and probably a bit less. Putting the second half in, I removed about 10 gal from the tank. Though I don't remember exactly, I believe I ended up returning about 3 to 4 gal of that. The rock I received is far more porous than some have indicated here. HTH