Best way was to just swap some old media with the new but I am assuming the new media is a filter cartridge and needs to do it your way. I had to do the same with my 2.5 cartridge and left it inside my other establlished filter for 2-3 weeks, but I bet you can wait just 2 or so....Really depends on the placement and if the bacteria can easily transfer. Can you put decorations oe even better, substrate from the 35 into the 2.5?