If adding more media, I personally would get K1 to mix with the ultima. Ultima has 3x the surface area but it is designed to hold crud inside of it until backwashed. I think the combo between the two medias would be the bee's knees. Although my bioreactor is non existent at the moment, I have done extensive reading. The crud at the bottom of the reactor will be mostly dead bio if prefiltering. The whole point of a bioreactor is to knock the older, dead/dying bb off of the media so that young, hungry bb can thrive. A reactor loaded with bio should have dead bb falling off of the media in mass amounts. An oversized bioreactor running properly does not need prefiltered but it will need the crap and dead bb removed regularly.