I also have fish in my tanks and tend to become overpopulated as new babies grow up and until I find new homes for them.
So this is what I have read:
Activated carbon adsorbs a number of dissolved contaminants such as; chloramines and chlorine, tannins (which color the water) and phenols (which cause odors), some heavy metals such as tin, mercury and iron; and fish medications. Indeed, thatís one of the best reasons to have carbon on hand: to whisk out any accidental overdoses or to remove residual medication after a disease treatment.
I have researched and found the best way to use it is about 3 tablespoons of activated carbon per 50 gallons. This will last about 1 to 2 days and then you can remove it and replace as you want. Suggestion was every two weeks. It needs to be placed after regular filtration and in a high flow area. It does not do well if clogged up with dirty water.