You need to feed it.Fish food, raw fish or prawn, muck from another filter system will will all help that. The time it takes will all be dependent on the amount of water you have, the filtration properties and how much you feed the water. Mine took about two weeks, to show "safe" readings, but I am not planning on adding fish for another week and then in small amounts. I.E 5 tetras a week for the next 4 or so weeks.