ahh, I see your question now... The way to maintain a quarantine when using a colonized filter, instead of an established tank, is to do frequent, small water changes -- 5-15% every 2 days is good. Constantly siphon stuff off the bottom (a bare bottom really works best in quarantine situations) and keep up with maintenance... An air-driven corner sponge filter is enough to maintain an adequate quarantine with a strict regimen of water changes

An established tank is a fantastic way to QT new specimen in a controlled, stress-free environment -- but comes with the established biofiltration (rock, etc) and thus if problems DO arise, medicating is no longer an option.

-Corruption