Im interested im buying a PH monitor, but dont know how much it would cost in teh long run in upkeep since you have to buy calibration powder? Does it not hurt the fish at all and how often do you need to use the powder?
pH measurement doesn't hurt the fish > you take a sample of water out of the tank to measure it, you don't (or shouldn't) put the probe in the tank.
Check ebay for probes, you should be able to find a good deal. The powder is used to calibrate the pH probe, how often you use it will depend on what the manufacturer of the probe recommends.