I'll start by saying that I have a somewhat scientific background, which includes some basic water treatment studies (I don't work in the field though, so I'm not an expert). It makes me a skeptic when it comes to many self-serving claims/studies made by manufacturers. Note that it does not make my opinions any more knowledgeable; I'll just argue them twice as much (or is it because my argumentative nature as my wife would say?)
Activated carbon removes organics and inorganics by adsorption and absorption. Adsorption handles organics by using electrostatic forces to adhere substances on the surface of the carbon (not to be confused with the ion exchange of resins), while absorption takes care of chemicals, inorganics in general and even trace metals in a way similar to a sponge (at least that's my understanding of it...there's my disclaimer).
Because activated carbon removes a wide range of compounds including chemicals (both organic and inorganic) it can be a very good tool. However, it isn't very effective after the first few days. Unless you have a specific reason to use it (like removing chemicals, pesticides etc from your tap water). In my opinion, it isn't necessary. But, it can be used with Purigen if you desire (I would not use Purigen while removing meds with carbon though). You could also use resins with Purigen without problems.
The reason why I say that carbon doesn't last more than a few days isn't because the carbon loses its properties in water, but because the adsorption sites are generally used up quickly on the small amount of carbon used by aquarists and thus, absorption gets blocked off too. It then becomes very good bio media (the bacteria will actually process some of the stuff taken in by the carbon). The person who told you that carbon can last much longer is right (I couldn't fool you )...but in my estimation, not for most tanks. You need to have a lot of carbon (much more than most people use) to counter the inevitable molasses/iodine numbers drop. I would have to research the subject to get semi-valid numbers, but many factors need to be taken into account (> 1 cup/10 gallons would be my initial guess since I heard that > 1 cup/25 gallons is recommended for saltwater tanks). That's almost a large canister filled with carbon on 125 g tank!!! Keeping it clean will obviously help, but most filters do a poor job at removing all particle (incl. the best power filters and canisters). I use floss before my Purigen and it will only last 1 month in my Eheims and AquaClears - Seachem claims up to 6 months - well not with fronts or other large cichlids and plecos (they don't clean anything...they are poop factories!). Anyway, In my opinion, using a micron filter inline (something like the Ocean Clears or better) before a large quantity of carbon could stretch the effectiveness of carbon by many months (commercial grade carbons for water treatment is sometimes rated for up to 6 months - that magic number again!). You would still need large quantities of carbon. There is, after all I know, very little valid aquarium research done on this, aside from the biased information provided by various carbon/filter manufacturers (I don't read the trade mags though). With many tanks like you have, it seems cheaper and less trouble to replace it every week (which is what I do when I need carbon). It is also pretty hard to see when a carbon is exhausted (unlike Purigen and certain resins). On another note, the price of a carbon doesn't insure it is effective.
Since carbon mostly removes organics (like Purigen), water changes and equipment service are much more important. Furthermore, depending on the tank and the characteristics of the water to be treated, carbon effectiveness will vary greatly (this I learned in school). I always ask why it is used, because depending on what is sought, other tools can be more interesting (like resins, or resin combinations - some of which including carbon). The extras it provides for growth, breeding etc is always good if it works for you, but not necessary. Hence similar results can be had with other products and can't always be duplicated (there's my disclaimer about Purigen...it works for me!). I might be completely wrong about the whole thing, but I'd love to see relevant research material.
If you do insist on going the carbon route, be very careful when you handle it. It gives off a very fine dust, lighter than smoke, easy to inhale and is not healthy to breath. Make sure you purchase bitmous coal carbon, you don't want coconut shell or medical grade carbon. They can't trap large particulates like bitmous coal carbon does. If anyone tells you that carbon robs the water of electrolytes then you may want to ask them why that's important. I have not seen any evidence why fish need to get electrolytes from the water when they ingest them in food, just like us.
Coal-based carbons (called Lignite) may be used for aquatic use. The molasses (macro-porous)/iodine (micro-porous) ratio is typically much better with this type of carbon to remove soluble materials (the numbers rate the size of the pores created by the activation). This type of carbon has ideal pores to remove soluble organics. Why other types are sold to aquarists is beyond me.
Carbon itself does not contribute to Hole-in-the-head (HITH) disease, the misuse of carbon may. Carbon is used extensively in water and air purification, and in aquaculture. Carbon is very effective in trapping large organic molecules like color, taste and odor, but has little effect on small molecules like nitrate and mineral ions. Carbon's effectiveness declines with age and soon reaches a "break through" point when the contaminants will pass through it unchanged. At that point, it will even release contaminants back to the passing water. The cleaner the passing water, the more wastes it will release, and so a water change in an aquarium without changing the carbon will negate its effect. In water treatment, it is a standard protocol to monitor the influent and effluent water quality daily to know when the break through point is reached and so new carbon is needed, but rarely do hobbyists perform that kind of monitoring. So the common misuse of carbon is not replacing it frequently and turning it into a waste reservoir and nitrate generator. HITH disease is suspected to be related to high nitrate level. The same thing can happen to a mechanical filter too, but one knows when to change a mechanical filter when it gets clogged up. But for carbon , the signal of when to change it is not obvious. with carbon filtration, the water may look crystal clear and yet loaded with nitrate and other dissolved waste invisible to our eyes. Carbon, if replaced frequently, is effective in removing nitrate precursors. However, for fresh water folks, the primary reason why carbon is not essential is that the simplest and cheapest way to remove nitrate precursors, odor, color and all other dissolved wastes is frequent water change. For salt water folks, though, it is both tedious and expensive to do frequent salt water changes, and so carbon has its place because it is comparatively cheap and convenient.
The only time I might use carbon is to remove medication, stain from drift wood etc. Be aware that the trapped wastes remain biological available to the bacteria which will ultimately break them down into nitrate, releasing them back to the water. Carbon should therefore be restricted to short term use. □