Water softener exchange tanks are rented from a local softener company and need to be changed frequently. Depending on the hardness of your local water, you will need to have the tanks exchanged 1 to 4 times per month. Rental cost per month is usually $25 per tank per month.
Exchange tanks offer the convenience of never having to add salt yourself. But there are many disadvantages to renting an exchange tank. Your water quality will change dramatically between tank exchanges. As the "salt charge" runs out, your water will be hard again. Your only option is to rent a more tanks more often, usually doubling your cost. As with all water softeners, you will also need to buy or rent a reverse osmosis system to filter out dirt and chlorine as well as the salt the softener tank adds to your water. If your exchange tank is stored in your garage or behind a fence, you will need to allow access to your vendor to exchange the tanks.