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I have just finished a seven-day treatment of the water with Melafix, and can put the filter (one with carbon) back into the water. I do have a lot of old filter carbon lying around, and I"m wondering if I can use that--or does it expire/lose its effectiveness? This stuff has to be years old.
I'm not talking about carbon that has already been in the water--I'm talking about stuff in small boxes that has just been laying around, waiting to be used in an aquarium.
It will not go out of date. As long as it has been in a box, without water or air actively flowing through it, it will remain active.
'Active' in this case means able to bond with chemicals, toxins etc. on a molecular level. Carbon permanently bonds chemically to all types of things you do not want in your aquarium. After all the surface areas of all the carbon is 'full', all of it having bonded to chemicals etc, it becomes useless. This all depends on the amount of chemicals flowing through it. (in some industries they even use activated carbon to clean and polish air, but this only works with the air being pumped though it.)
Now, activated carbon will NOT deteriorate in a box. You are totally ok to use it.
If you ever go to buy more, make sure it is 'activated' carbon, and not just carbon. Activated carbon has been superheated in ovens to about 8000 degrees celcius or some other ridiculous temperature. This makes it about eight times more effective (it makes it more porous...) than regular carbon, which is not different from charcoal from your fireplace....
In my aquarium, I replace mine just about every month, but I have a heavily stocked tank with a lot of food, and a large filter that works hard. I also have access to cheap replacement carbon 
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