Thanks to a PM I received,
here is a cheaper way to test for Magnesium using hagen's test kits
Use the present Hardness test kit in the following procedure to obtain a measurement of the magnesium level in salt water.
1. Do a modified Hardness test for Saltwater
Saltwater 5 %
Measure 5 ml of saltwater from the aquarium in question and dilute with 95 ml of demineralized, distilled or reverse osmosis treated water.
Pour 5 ml of this 5 % solution into the test tube provided with the Hagen Hardness Test Kit (Art. # A-7830).
Use the GH test kit directions, however in the calculation replace the factor of 20 with a factor of 400. This provides the amount for A in the calculation below.
2.Analyse for calcium
Use a sample of standard saltwater from the aquarium (not a diluted one)
Follow the directions according to the leaflet. The result of the test should be multiplied by 2.5 to provide the proper amount for B in the calculation below.
3.Use the calculation included on the back of the Calcium booklet to determine the actual magnesium concentration.
A - B = Magnesium mg/L
4.118
A - General Hardness measurement
B - Calcium result times 2.5 (calcium result X 2.5)
Normal magnesium levels of saltwater should be between 1300 and 1400 mg/L.