I bought a hand-held probe a little while ago and was wondering what is the difference between the different calibration fluids. Mine came with a 4.00 and a 6.50(?) calibration fluid bottle. On Bulk Reef, they sell the 4.00, 7.00 and 10.00 fluids. What are each one used for? Different temps?? Which one should I get through the group buy? My probe didn't really come with any good directions and it was cheap... maybe a mistake on my part, but it works pretty well for now.