It depends really on what levels of nitrate your tap water has. The lower the better really, and as long as your water changes are large/frequent enough that it's not building up more than you're removing it, it's fine. 100ppm is a maximum you want to work to, though ideally you want much less than that. Most fish can easily handle 100ppm, some fish can handle much more than that, but 100ppm is a maximum to be safe.