I believe they maintain a stock of cycled filter media and substrate as well as adding bacteria to facilitate the cycle. There was an interview Mark Callahan did with Brett from the show that said they would try and talk about some of the technical aspects of the process in season two (the current season). I haven't been able to watch any of it yet, but it's on my DVR. I imagine in most cases they are also sellign their customers a maintenance contract as well.