Well certainly the 2 best kinds of filtration to have are Biological and Mechanical.
Your undergravel filter provides excellent biological filtration, but sometimes can be a pain to clean and maintain. What type of filter is your power filter? Do you know the specs of it, like how many GPH it filters? Usually it's best to have a filtration system that filters your entire tank at least a couple times in an hour.
Carbon is mainly used for clearing the water of medication, but I found a list of other stuff carbon removes on a website and thought it was pretty interesting.
Absorption Potential of Various Substances by Activated Carbon
High to very good
Arsenic, bleach, chloramine, chlorine, chromium, colors, dyes, gold, insecticide, odors, monochloramine, tin
Good to Moderate
Acetic acid, cobalt, detergent, hydrogen sulfide, mercury, ozone, potassium, silver, soap, solvents, vinegar
Copper, iron (not chelated), lead nickel, titanium, vanadium
Low to None
Alkalinity, ammonia, barium, carbon dioxide, hardness, copper, manganese, nitrates, selenium, molybdenum, zinc.
It's a debatable topic on whether carbon should be used regularly.