From The Aquarium Wiki
Ion-exchange is a process where an ionized solution present in large numbers or with a strong charge takes the place of another ion that is attached to a matrix. So they effectively swap place.
This is the process used in many aquarium products that use resin to remove impurities and chemicals from water.
Typically a resin exchange may remove nitrogen waste (ammonia, nitrite and nitrate), metals, phosphate, silicate, calcium or magnesium ions from water to lower its general hardness (GH) therefore to make it soft. Some resins can be partly recharged by running a very strong salt solution through it for an hour or two. But the process is never 100% efficient and eventually the entire resin needs to be replaced.
Knowing the type of exchange
When buying or using a Ion exchange device it is important to know what ions you're replacing. If it is swapping, for example, calcium or magnesium for sodium ions to reduce GH then this may kill most South America tropical fish as they are sensitive to levels of sodium.
So choose a ion exchange device that swaps calcium/magnesium ions for hydrogen ions in this case. This will also lower the pH as well of course.
Types of Ion Exchangers
- Mineral exchanges - For example: Zeolite, Carbon, kaolins and other clays.
- Synthetic exchanges - For example: Purigen or PhosGuard