well i dont get how your biological filtration could be failing you with this info. unless ofcourse you rinse your carbon out and reuse it. However, there is no need to use carbon on your tank. try using pillow stuffing instead. The thing is that carbon pulls out all types of chemicals.. good chemicals and bad ones.

What kind of filter do you use? fluval fx series? rena xp? hob? and what are you using to house your bacteria?