I kept everything in the tank. I had a six-line wrasse, frogspawn, torch, hammer, Duncan, favia, zoas, rics, hermit crabs, an emerald, and some snails. I just moved all the rocks to the bottom of the tank so they wouldn't fall and took out about 3-4 gallons of water, so 3-4 jugs. Just take out as much water as you think is necessary so it doesn't flood your car when you hit your breaks or take a sharp turn.