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. Most medications state that the carbon should be removed during treatment. We have had several cases when we were asked for help with an aquarium problem, which was caused by not replacing the carbon when treatment was over. It s easy to forget, so make sure you leave yourself a reminder. Using carbon in planted tanks might deplete some trace elements needed for plant growth.