This is where a bio-wheel would come in handy. A well established bio-wheel can adjust to changes in the bio-load much better than a submerged sponge (or any other submerged filter media) can. About a year ago, I had to move about 20 white cloud mountain minnows from one tank into a 20 gallon that already had about 10 - 12 fish (filtration done by an emperor 280, and an aquaclear 150 with filter floss for extra mechanical filtration), and there wasnt any problems at all. If I wouldnt have had the emperor 280 with an established bio-wheel on the tank, my ammonia and nitrite probably would have went through the roof. Obviously adding that many fish at once isnt a good idea, but I had no choice at the time, and thankfully it all worked out.