Yes, run the powerhead the entire time the water is mixing. Adding the salt mix raises the pH of water--the mix includes buffers (calcium carbonate, etc) to increase pH and alkalinity. How high is the alk?
I've never used the Crystal Sea salt mix, so can't say if this is typical of their mix or not. From an content analysis, it doesn't seem to contain anything that would cause a high pH or other complications, but as with all salt mixes, it has it's fans and detractors--some claiming that it's lethal in their tanks. However--most reports of problems I found were after individuals switched brands, and that can always cause problems, no matter what brands are involved.
I've never used the Crystal Sea salt mix, so can't say if this is typical of their mix or not. From an content analysis, it doesn't seem to contain anything that would cause a high pH or other complications, but as with all salt mixes, it has it's fans and detractors--some claiming that it's lethal in their tanks. However--most reports of problems I found were after individuals switched brands, and that can always cause problems, no matter what brands are involved.