You need to recognize what you are working with Josh. In an undergravel filter, the gravel itself is the mechanical and biological filter. It can get a jump start the same way as any other filter. You can put some mature media into the tank so that water will go through it on the way to the new filter. In this case it means laying some filter media on the bottom of the tank right on the gravel. When you cycle that filter using a bit of ammonia, the bacteria on the media will set up shop on the gravel substrate and you are in business. I have done the same thing with a sponge filter. A sponge has no more place inside for mature media than a UGF but I laid a couple of pieces of media against the outside of the sponge and the filter was ready to use in a very short time.
