There are some types of salt that you can and some types of salt you cannot use in a water softener. Let it be said that any time you're looking for salt for your water softener you should be looking to buy water softener salt. It's intended for that purpose so it makes sense to use it. But there are a few reasons you may need to look to an alternate:
you forgot to buy softener salt and you need something ASAP
you've got another type of salt you're looking to use up
softener salt isn't available in your area because of supply chain issues or a strike
you've found something cheaper than regular softener salt and you'd like to use it to save money
Below are the different types of salts we've been asked about over the years. We do our best to explain what they are and whether they're appropriate for use in a softener. If they are useable we go over any potential risks for each one.
Rock salt is mined from the ground and barely processed. So it's usually a combination of salt, dirt, sand, and small pebbles. It's commonly spread on highways and roads as an ice melter in northern climates. Because it's not processed very much, it's cheap. Rock salt can be used in water softeners as long as no other "ice melting" chemicals have been added to it. However, because of all the non-soluble impurities it contains, rock salt will leave a muddy deposit in the bottom of your brine tank which can make it smell. So although rock salt will work in softener, it's not a great choice. If you decide to use it, you really need to make sure you have a softener that has a detachable brine tank as opposed to an "all in one" design. That way you can take it out in the yard and hose it out every year.
Ice Melter Salt
Have you got some leftover ice melter salt that you'd like to dump into the softener? Don't do it! Ice melter salt contains toxic chemicals that you don't want in your household water. Using ice melter in a softener is a definite no-no.
Yes - you can use table salt in a water softener. Both the iodized and non-iodized forms will work fine, as will coarse table salt and kosher table salt. The only thing to watch for with table salt is that the fine grains can make their way into some of the mechanical components of the brine tank. If these grains get into the safety float water intake they make prevent the air check from closing properly. You will know if you have this issue because you'll have air in your lines after the softener regenerates. This is a temporary issue though that can be solved by swirling some warm water in the brine tank to dissolve any remaining salt.
We've had a few farmers and ranchers ask us over the years if they can take an old salt lick used for livestock and break it up and put it in the brine tank of their softener. The problem with salt licks is that they are very rarely just salt. To the best of our knowledge they usually contain something else. This may be vitamins or minerals, sugar, dye, fruit juice, or other "attractants" (especially for salt licks used to attract game animals). So while we know that a salt lick will technically work to regenerate a softener (our customers have told us, lol), it's not recommended at all. It's hard to know what the other ingredients are going to do to the softener, or to the people!
Some pools require the addition of salt which is converted into chlorine by a special machine in the pool filter. To the best of our knowledge, pool salt is just pure sodium chloride (NaCl) and as such should work just fine in a water softener if the need arises. So just make sure there aren't any unusual additives and you should be good to go. One thing to note is that pool salt is oftener "finer" than water softener salt and may work itself into the brine tank air check valve. This isn't the end of the world but it may result in air being found in your plumbing immediately after a softener regeneration.