pH Drop Test Kit
$6.95

From Aquarium Pharmaceuticals -- pH readings fluctuate during the day so pH testing should be done at several different times.  Pond fish prefer a pH range of 6.8 to 8.0.  Less than 6.8 means the water is too acidic / More than 8.0 means the water is too alkaline.  A pH crash can kill every fish in the pond.  Prevent fluctuations in pH by using a
pH stabilizer.