How can I bring the nitrite levels down in my tropical fish tank?

My aquarium is about 10 days old and has had fish in it about a week (I used Cycle to mature the bacteria levels quickly so I could add fish after 2 days). The nitrite levels are showing as 'harmful' - ie not good over long periods for the fish. How long will it be before the 2nd type of bacteria kicks in to convert the nitrite to nitrate? And is there anything I can do to speed it up? The 4 fish seem to be ok, perhaps breathing a little quickly.