Test the water. This happens when the major elements are not in/near NSW proportion or quantity. Non NSW parameter salt is one reason, but if you put in all of the salt with not-enough water (even if you are going to add it later), it can precipitate. You can also get this from batches that are off-the-norm from the manufacturer.
To avoid this, use a reef-quality salt, add salt to the full volume of COLD water and heat after mixed.