I use one for my calcium reactor, and one for the tank. Currently My PH is "low" 7.8 according to the old hanna meter, but my alk and cal are 9.2 and 400. Sooo I still feel the need to keep an eye on my PH. I've had the hanna for 6-7 years, and its not holding a calibration like it did when it was new. I'm just looking for opinions on if these hand held meters are worth $40, or should I buy the $100? Will the $100 meter really last a lifetime?