No you do not need to leave it at 0ppm this would be nearly imposible. You do not wont to add fish until the cycle is over you would just be signing their death certificate if you did. I sugest durring cycling with fish a 50% water change daily. This will keep your levels down a little. Only use hardy fish IE. Feeder gold fish as they are cheap and hardy and also usaly so cramped already thous being used to the high levels. Good luck and consider some sort of starter culter for your cycle.