There's two reallly good reasons to never use hot water while mixing your salt... Copper and Lead. In older houses, the hot water pipes often contain lead. And all hot water heaters have copper cores and copper plumbing. Copper being very inimical to your fish, of course. And no, an RO
is NOT enough to get rid of copper. It's much safer to use cold water and throw a heater into your mixing bucket/can/whatever to bring it up to temp than it is to use hot water. I have a garbage can I fill with freshwater from my RO
during the week, with a heater on in it all the time, and I add my salt about 36 hours before I do a water change so it has time to mix and stabilize. I use a Rio junk-pump 2100 in the can to turn the water over, and have no issues with cloudiness or undissolved solids. And I use a local brand salt, Pacific blah blah (Coast I think?) with great results, far superior to Oceanic which gave me serious ph problems and a weeeird outbreak of diatom-like stuff.