Activated Carbon, purigen, are these useful for planted tanks?

Activated Carbon & Purigen

These two fall into the category of water polishers, capturing small particles that are invisible to the eye, and both help to improve water clarity and adsorb organic molecules.

Activated carbon is used to absorb odours and tannins and is good at adsorbing larger molecules - usually meaning organic compounds with long molecular chains (such as pharmaceuticals). Contrary to popular belief, it will not remove most fertilisers in a planted aquarium as the molecules are too small to be captured. The fertiliser it does remove is chelated iron, which is a large molecule consisting of iron (Fe) attached to a chelating agent, such as an organic acid in ferrous gluconate. However, if dosed regularly, the effect of activated carbon is insignificant - so you can use it if you wish. The list of what activated carbon removes effectively is at the bottom of the page.

Is activated carbon necessary in most planted aquariums? No, but it can be useful in setups to trap small organic waste particles or tannins.

Purigen is a good alternative as it can be recharged (with bleach) and reused. It polishes the water to a very high level of clarity by capturing small particles that are too coarse to be captured by the filter.

ADA Gallery in Niigata, Japan, often uses activated carbon to remove organic waste and tannins, especially in newer facilities. These techniques have produced some of the most successful, stable, long-term tanks in the industry - clearly dispelling the myth that carbon is detrimental to the operation of a planted tank.

PhosGuard, de-nitrate, Zeolite and other ion-exchange resins

These are not necessary in a planted aquarium and will not help solve algae problems. Plants need a certain amount of phosphates and nitrogen to grow well and these resins run the risk of completely depleting the water of the necessary nutrients. If your tap water is within the limits of what is safe for human consumption, there will generally not be such an imbalance of elements as to cause serious problems.

List of elements that Carbon capture effectively;

Here's a table that gives you a good idea of what is removed by carbon.

High to moderate adsorption on:
Arsenic, Bleach, Chlorine, Colors, Dyes, Hydrogen Peroxide, Insecticides, Monochloramine, Odors (usually larger organic molecules), Detergents, Dissolved Organic Compounds (DOC), Hydrogen Sulfide, Mercury, Soap, Solvent

Fair adsorption on:
Complexed trace elements (due to organic chelator), Iron(as FE 3+), Lead, Vanadium.

Low to no adsorption on:
Alkalinity, Ammonia, Carbon Dioxide, Nitrates, Phosphates, Potassium