The amount of carbon that should be used varies from one aquarium to another. Anywhere from 3 tablespoons to one cup per 55 Gallons is generally recommended. The frequency of replacement is also in large dependent on each individual tank size and setup. Replacement may be required as little as every 6 weeks to as often as every 12 hours. As mentioned above, the yellowing of the water is an excellent indicator as to when the carbon should be replaced.
Oxygen depletion is caused by the use of too much carbon.
You can avoid this by presoaking the carbon for an hour before placing it in the aquarium filter. Some carbon products leach out phosphates . You can determine whether the carbon you use leaches by soaking carbon in some water and measuring for phosphates several hours later. Carbon readily absorbs most of the aquarium medications, especially antibiotics. Keep this in mind, while medicating your tank.