Popular opinion around here seems to be mix your SW a week ahead, or when you do a water change, go ahead and mix a batch for your next one. Just keep a pump/powerhead in your container moving the water around and a heater if necessary.
To get a good read on your mixture, you should wait at least an hour before measuring your SG.
I don't know about the anemone but I've heard that they turn to mush when they die and you really want to get it out of there. If it died, there may be a chance that he fouled your water and caused the other deaths. How big is the tank? What are your current water parameters?