Baffled by calcium test discrepancies

[PoonTang]

I seriously doubt your Cal was ever anywhere near 600. With you mag and Alk being that low it most likely would have been snowing in your tank. How heavy is your Cal demand? ie. how many sps colonies in your tank? You are using a Caddilac mag test kit but a hyundai cal kit, toss it and get either a Salifert or Elos as they dont go by colour gradient but by a rather definate endpoint.