major overhaul

Hi all. I am moving my 65 gal tank from one room to the next and am wondering how much water to reserve. It's a 65 gallon tank with a 20 gallon sump. The sump is perhaps holding only about 12 to 14 gallons of water because of not being filled to the top and holding equipment. The 65 gallon has about 20 pounds of rock and very little sand. Would 20% to 25% be sufficient? I'm expecting this to take several hours and am worried about some bacteria loss if the rocks sit in non-circulating water for too long. Should this be a concern?

Thanks in advance.