you cant is the answer, you need another smaller sized tank, you could try medicating the whole tank, i use myxacin made by waterlife sold in the uk, maidenhead aquatics also use it, its a broad spectrum antibacterial. it isnt supposed to damage the filter media or plants.
or buy a small cheap acrylic tank with a small filter heater its best to keep it running and matured all the time.
good luck with the krib
