Continuous is tricky. I know ther used to be a color-changing pH monitor thing, but that only lasted a month at a time.
For pH I know you can
buy a probe and amplify the output, but for nitrates, nitrite, and ammonia continuous sensors are not available.
The only solution I've seen for continuous testing of those is a sketchy setup that involved using two dosing pumps to add water and reagent to a vial and measuring the color with a spectrometer to find your concentration.
You probably don't need to measure nitrites, if that's any consolation. Same goes for ammonia, because you can always test for that if something looks off, and a properly developed aquarium will remove ammonia pretty quickly.
I have had good success with liquid tests for accurate readings, although tedious. Is this for saltwater or freshwater, and what size tank do you have?