Its always been at the back of my mind but my tank has always correlated with the the test results. If anything was off, the tank showed it. There are formulas online to make your own reference fluids to test against but it usually necessitated hunting down chemicals and having a precise enough measuring equipment to make sure that your reference fluid is indeed of that concentration. And you can't use the instrument you are going to check to check if the fluid is made correctly. The other thing that I pay attention to is if my testing procedure is correct and I'm not introducing elements that impart another variable into the test. While browsing BRS during the May group buy, I came across a Multi Reference Solution by Fauna Marin. I purchased it and used it last week with the whole battery of my test kits. I am happy that all my test kits and testing protocols have returned results well within the tolerances the test kits provide. Some read consistently low, some high, some spot on. I've taken note of which are different and will make a note of when recording the numbers. Parameters that can be tested Ca Mg K Salinity Alkalinity SiO2 Si(OH)4 Strontium Si Nitrate PO4 It covers a pretty good range of parameters that are relevant to home aquaria. The units are metric so there is a little conversion you have to make but its very easy to look up. Most are in mg/l which is very very close to ppm and for all intents and purposes they are equal. Alk is in ppm. Overall I like the product and will continue to use it. I plan on using it whenever I start a new pack/bottle or reagent and if the results of my regular tests seem suspect. http://www.bulkreefsupply.com/fauna-marin-multi-reference-solution.html#tab-full-details If you try it, let me know how it works for you.