I have a digital pH tester which is giving me different results than my Freshwater Master Test Kit from API and I'm not sure which to trust.  My digital pH meter measures my Standard Reference Solution of 7.0 and 4.01 spot on each time.  The same digital meter when I measure the water from my aquaponics system reads 6.6 while the Freshwater Test kit is saying my water pH is around 7.6.  I'm wond…