Kalk paste has two risks ya just need to know. First, depending on the size of your tank and how much you're going to nuke, be mindful of the potential ALK spike as Kalk has somewhere near 14dkh. Second, it's kind of like a grenade - once you turn pumps back on, inevitably there will be parts that don't dissolve, and will kind of go around. Be aware that if a chunk lands on anything alive, it will burn it.
Another option, assuming you can tak the rock out, is sell the rock and replace it.