When I used premixed kalk I stored it in my top off container, a 5 1/2 gal fishtank. I covered it with saran wrap and the kalk would last for the 10 days or so between refills. I'm moving to a reactor for convenience and safety.
Jeremy, your question is a good one and one that a lot of reefers should know the answer to.
Using kalkwasser for topoff is a good way to replenish your Ca, if done properly. If done the wrong way you can wipe out your tank. The reason kalkwasser reactors are used is so you can stir the mix. The 'sludge' at the bottom of the container you store your kalkwasser mix in is not to be used unless it is mixed and floating in suspension.
Different kalkwasser reactors use varying methods for stirring the mix. They all look to do the same thing though.... in the end they all try to mix, stir, swirl, suspend, shake, rumble, tumble, etc. the kalkwasser in water. Some reactors do this with timers also. The timer would come on a pre-set amount of time before the topoff refills the sump with water.
The bottom line is this, the topoff water with kalkwasser in it should not be clumped at the bottom. If it is clumped, you can use the water from the top of the container. Just DON'T USE the clump.
I use a 15 gal TO container with a pump inside that is elevated off the ground so it dosent suck up the sludge. I also have a powerhead in there but only use it to initally mix the kalk. I then wait a coupke of hours for the solution to settel and plug the ATO back in. ( i make sure I top off whatever evap'd during the mixing period so I dont dump a bunch of solution in my tank all at once)