Fauna Marin has made a Multi Reference solution in a 100ml bottle to test the accuracy of any test kit. It is a calibrated solution with specific concentrations of Calcium, Magnesium, Kalium, Salinity, DKH, and Silicates. Instead of using your tank water you use this solution and complete a test. The results will tell you how accurate your test kits are!
http://www.reefperfection.com/index....ROD&ProdID=469