Ah. Instead of replacing it, rinse it in clean water that's been dechlorinated to remove the gunk, and keep using it. This is where the beneficial bacteria live, and by replacing it, you remove those colonies, forcing the tank to recycle. This is not good for the fish--it exposes them to high levels of ammonia. I'd advise going with a different media--the carbon isn't required, and you'd be better off with a sponge or some type, or cermaic noodles, that provide lots of surface area for the bacteria to colonize. But--the change over should be done gradually, so you don't pull out the enitre colony of beneficial bacteria all at once.