No such thing as a Nitrate Probe exists except for an Ion probe which is not going to work continuously as it will quickly drift off within minutes. Ion probes must be calibrated just before use and then removed from the water right after they are used. I have not seen one yet for Aquarium use but the upcoming GHL Ion Director will have one so you may be able to get a reading done a couple of times per day. I really wish that a continuous probe existed! The closest possible thing might have been the Mindstream which is not on the market anymore. They did say that it might be possible to use the florescent dye disks to take readings every 15 minutes but this was only something they mentioned as a possible future feature.