These things sort of exist for the hobbyist but nitrite and nitrate are not available and that is really what you are after. You can get probes to constantly monitor TDS, PH, Temp, I am sure salt water folks have more to measure as well. Seachem makes a little sticky to put in your tank called an ammonia alert, but really, if your tank is cycled you should never see ammonia, same goes for nitrite. Nitrate is the one you are going to see climbing and dropping after a water change. This product looks close to what I would want, not sure if works for freshwater. Also I would like it if it logged the data and could be downloaded or sent to a computer to chart it. https://www.amazon.com/American-Marine-Pinpoint-Nitrate-Monitor/dp/B001EHAYUE