But do not use destilled water for keeping, but only the advised keeping sdolution. Also some manufacturer advise to clean the electrodes with tap- not destilled-water.
That is important advise. The problem with the cheap $10 all in one test stick style pH meters is that you actually can't keep the electrode in a storage solution. Maybe the quoted one from Hanna is different there.
I'd recommend a unit with external probe. They are easier to handle and the probes come with a storage tube that is filled with KCl solution.
Another thing for us photographers to keep in mind is that most pH probes must not be used with solutions that contain more than small amounts of Silver ions. The reference electrode in the probe is a Ag/AgCl element immersed in KCl. Maybe Ag+ in the measured solution will precipitate in the porous junction connecting the KCl with the outside.
The maker of my equipment used to have a special probe with KCl and KNO3 electrolyte (probably inner and outer electrolyte with an extra junction in between), which was offered for use in photographic labs. It could be used with Ag+ loaded solutions. But I guess that is another victim of the implosion of the film market.
Monitoring the pH of increasingly used fixer is not advisable for that reason. I do wonder about used colour developer, though. I'd expect it not to wash out any Silver from film/paper. Does anyone know for sure if I'm right? What about rehalogenating bleach?