I freeze the leftover tuna juice until the next time I need it, which is about a month or so later, depending on how much I soaked at that time, I have a big container of ZooMed's turtle food plus the smaller one from before, I soak enough to fill the smaller one about half way, that way if for some reason the food spoils, I won't lose so much of their food.
I also put some of the calcium supplement in with the tuna juice so that when the turtle food soaks in the tuna juice, it will also soak in some of the calcium supplement.