You can either leave the drop checker in the tank, or take it out, drain the fluid out of it, and set it up again whenever you want to check the CO2 again. Since the latter is a lot of work, why not just leave it in the tank? You could put it in an inconspicuous location in the tank, then move it out where you can see it easier when you want to use it. If you left it out of the water very long the solution in it would begin evaporating, which would raise the KH of the water in it.