Depending on what type of electrode you use the readings can drift quickly and the electrode can wear down over time. I'm sourcing solid state nitrate and ammonium electrodes from a manufacturer that are showing good consistency. The main issue is how often it needs to be calibrated and that depends on the accuracy you are looking for. They also have replaceable cartridges so the entire sensor wouldn't need to be replaced when it wears down. The electrodes are pretty affordable as they are the half cell type where they'll go for 400. I've been looking into this for aquariums as well and it seems like for that application people aren't as willing to calibrate on a regular basis. Would calibrations every two weeks be too much of a hassle? The frequency of calibration may be different than that depending on what type of accuracy you are looking for though.