Thanks @paulraphael, I have the SousVide Dash app but I don't have much experience using it. I see I'll have to do my homework!
Tonight I heated the water to 118F. Added the bagged salmon and some boiling water. Thought that I added too much hot water, so I added some cold. Thought I added too much cold, so poured in some more hot. Temp settled down to about 116F, eventually getting back to 118F. Cooked the bag for 22 minutes.
Just divine.
Now I have some halibut to try. And some spot prawns. The halibut is over 1" thick so I'll have to adjust my ideas a bit. Any ideas about time and temp?