Carbon is “activated” by adding a positive charge UV water sterilizers, which enhances the adsorption and reduction of contaminants which have a negative charge. The three forms of activated carbon used in water filtration systems are granulated activated carbon (GAC), activated carbon block, and catalytic carbon. Activated carbon removes impurities, chemicals and contaminants from water through adsorption and a process called catalytic reduction. Contaminants removed include volatile organic compounds (VOCs), herbicides, pesticides, chlorine (which is why water tastes better when filtered with activated carbon), chloramine, radon, and most man-made chemicals including MTBE a chemical used to improve the octane in gasoline while helping to lower emissions. Activated carbon is not effective at removing heavy metals, nitrites, nitrates, dissolved inorganic contaminants or sediment. This is why sediment filters should precede carbon filters, and why KDF media is combined with activated carbon in many quality filtration systems. There are three basic types of UV sterilizers on the market today. Module style, Multifunction, and a few "Twist type" models. When considering UV in your filtration system I would suggest looking at your system as a whole and deciding what style would be a good fit for your aquarium. Consider model design, and also look at the size of the UV and what extra pumps, bulkheads, and shut off valves will be needed to make it easy to maintain and not be in the way of your other Components. Module style, (as pictured) above typically are free standing uv ballasts, some models can be mounted on their side as well for better placement in your system. The idea behind these models is you can add fluidized bed filters, heater modules, and many other things, link them all together and power everything with one pump.