I tried the Red Sea Iron test and never got a good result. The test tube turned green, and the color chart included red, blue and grey.
Even when I over dosed and tested soon after, so the iron was well circulated.
What is the GH of your water? This is a combined Ca and Mg test. It does not tell you either one separately. If it is high enough I would assume there is enough of both in the water. If the level is under about 3 German degrees of hardness, though, it would be good to test and see if you need to add one or the other (Ca or Mg). Similarly, if the plants seem deficient in something then knowing the GH and Ca and doing a little math can tell you what the Mg is, also, and help you figure out what to dose. The math is not as simple as subtracting one from the other, look it up.