How fast do potassium sorbate and camden tablets work?

Pretty much the topic. I have a mead that I am about to rack to secondary. In secondary, I am going to add blueberries, vanilla beans, and more honey. I don't want the yeast to ferment any of the sugar I am adding.

So my question is should I add the p. sorbate and camden tablets into the primary and give it a few days to halt fermentation and then rack? Or just rack on top of the sorbate and camden tablets?

Also, Should I mix this stuff with water first or just add the powder into the carboy?

They will not stop active fermentation, just prevent new. If it is nt finished and you add ferment-able sugars it will start again.If you get a consistent reading with Hydrometer for 3 days in a row, then add campden and sorbate. I would wait 24hours after adding stabilizers before sweetening