When i upgraded tanks, I tried to keep as much as water as possible, so instead of a 100% water change, it ended up being more like a 25%. To do this, I caught the fish and put them in a bowl. Then i took my gravel vac and started it running WITHOUT cleaning the gravel, just using the vac as a siphon. I then refilled my tank with dechlorinated water, then cleaned the gravel. Moved everything to my new tank, added more gravel, then started to refill the tank. I also put the old filter spounge in the new filter to keep the bacteria and alive. If u leave the spounge in your new filter, it will keep bacteria on it, whihc can be good if you are using your old tank as a quarentine tank. Hope that helps!