Not all carbon is created equal. It all contains phosphate but high quality carbon made for aquarium use, has the least amount of phosphate.
The cheap stuff is usually made for air filters, and it's re-labeled for aquarium use. Some has small pores that isn't that effective.
And there are some studies that link carbon fines to HLLE... that's a whole other thread.
Carbon is good, for the most part, but the kind of carbon you use does matter.
Jenn