fishdudein
Fishaholic
Whatever it takes to get the beneficial bacteria "started" in your new tank. Did the driftwood come from your old tanks? If so, you have already imported a good source of bacteria into your tank. The sponges would be another good idea. If you don't want to mix two different colors of gravel (what I interpreted you as meaning), then the gravel in a sock should work- I have even put some of my old gravel into a ceramic or crockware bowl and set that in my aquarium. Plants, decorations, anything from your old tank with a surface area that bacteria can grow on will help but the filter media and sponges with their large porous surface area are the best sources.