I buy it by the box, cheaper and easier in my opinion.
When you get the 200g box it has four smaller bags, which are perfect to fill up a bucket and use til the next it runs low. Keeps the salt from clumping AND it allows you to mix the bag up. During shipping the heavier elements in salt mix will sink to the bottom of a bucket, like strontium, which will result in low readings for those elements at the beginning of the bucket and by the time you are down to the bottom some readings will spike.