If I remember correctly, after bringing home 30 gallons and getting my bc29 filled with just the substrate in the tank, I had 6 or possibly even 7 gallons left over. I let the filtration run for a day or two, while correction salinity, before adding live rock. Dont forget that the 7 (or so) pounds of salt you'll need to add will take up some volume also, so you might just make it with 24 gallons.