Most fog machines that are cheap have a built in fluid tank so you will have to empty it. But wait, don't throw away the fluid. If the fluid is still clear (or it's original color) and doesn't have any debris in it, you can use it again. I recommend getting a small Nalgene container to store the fluid in. A 500ml container is usually about the right size and should only cost a couple of dollars at a camping / outdoors store. In Canada we have Mountain Equipment Co-op and Campers Village which are the best places to find a container like this. As a side note, if the fluid is clear but has debris in it, you can pour the fluid through a disposable coffee filter to remove the debris.