Chemical media can target several issues. Here is a brief list of the more common chemical media.
If you have this problem- then try this media:
Ammonia- Zeolite
Excessive nitrates from organic waste, tannins, a few other things- Purigen
Misc. toxins, especially organic things like medicines- Activated Carbon
High GH- GH softening pillow (though this is better done before the water is added to the tank)
Copper, high Phosphate or a few other single element problems- Cupri-sorb, Phos-sorb and related materials.
Water has too low GH, KH, pH- Coral sand, Oyster shell grit or related materials
Want to add tannins- peat moss.