The most serious problem limiting use of ion-selective electrodes is interference from other, undesired, ions. No ion-selective electrodes are completely ion-specific; all are sensitive to other ions having similar physical properties, to an extent which depends on the degree of similarity. Most of these interferences are weak enough to be ignored, but in some cases the electrode may actually be much more sensitive to the interfering ion than to the desired ion, requiring that the interfering ion be present only in relatively very low concentrations, or entirely absent. In practice, the relative sensitivities of each type of ion-specific electrode to various interfering ions is generally known and should be checked for each case; however the precise degree of interference depends on many factors, preventing precise correction of readings. Instead, the calculation of relative degree of interference from the concentration of interfering ions can only be used as a guide to determine whether the approximate extent of the interference will allow reliable measurements, or whether the experiment will need to be redesigned so as to reduce the effect of interfering ions.NO3, NH4, Ca and other ion selective probes have serious issues with interference ions, that's why these are used for labs, not environmental monitoring................when you have high levels of other similar charged(say NH4+ and K+) or stronger ions present.
Generally, you take a sample, add other chemicals to remove/prevent this interference, then take the reading. The chemicals required to be added tend to rather nasty, so you are not going to add them to your tank. The calibration for the probe must also be done each time you use it. If you just leave it in the tank, the accuracy will float all over the place.
Here's a bit on it.
You also need a 1 or 10 ppm and 100, and 1000 ppm calibration solution set to calibrate the probe.
Having said all this, they are great for measuring lots of samples all at once.......so say you have 100 x 25 mls NO3 samples...........something aquarist rarely ever do...........but research very often does.
They ain't cheap either, so many research folks make their own.
One thing aquarist can do is freeze their water samples, and when they have 100 or so taken over some time frame(say daily for the last 3.5 months,), they can thaw and measure them and then plot the data with the known amounts of added KNO3 stock solution to their tank.
Few aquarists will EVER do this.
They just want something they can glance at and that's cheap that does not require a test kit method.
There are quite a few trade offs, I think it's far far cheaper and easier to simply automate your water changes. Dosing is very simple and accurate that way.
Then you do not worry about all this.