I recently bought the Profilux 4 Mega set and installed it in my aquarium. The initial setup and probe calibration went with ease. The only problem I have is with the conductivity probe. The reading after the calibration is spot on (checked with calibrated Hanna and refractometer). But shortly thereafter the reading keeps dropping. None of the probes are anywhere near any power cables and I have calibrated the conductivity twice. Am I doing something wrong?