If you switch, I would say the transition time depends on your fishload, number of plants, and how often you do water changes. So, as far as weaning time, I don't know, a couple of weeks should be alright. I would think most of the nitrifying bacteria is in the filter's biowheel, sponge or whatever. On the penguins, I think you can modify those filter cartridges so that you lose the carbon and cut filter padding to fit and secure with a rubber band or something. Much cheaper. Some just rinse the old padding of the clogging detritus in tank water from a water change and reuse. That's supposed to save the good bacteria.