5ppm shouldn't kill you or the critters. Nitrates are usually not toxic, but rather indicative of high levels of other parameters we can't test as easily (ie, dissolved organics). Keeping them below 10 is usually acceptable for a reef system, and below 20 for a FO. We have xenia, so our nitrates never get very high--meaning it's not as valuable a tool in determining if a water change is needed.