Well in that case...
Basically you plan ahead how many tanks you are going to do in one session. For that, you will have a rough idea how much water you are going to use. So you could dissolve the amount of Bee Mineral which you will need for your task.
Say in one tank you are going to change 20l, so you prepare a glass of your changing Water with the needed amount of the mineral. You do, say, five tanks... it means you prepare five glasses of water, ready to be used and to be tipped in the tank.
Get away from the idea that the mineral is supposed to be fully dissolved. The undissolved Minerals are usually rock Minerals and will only react if needed. That is one of the reasons, why you shouldn't mix a big portion.