Not all activated carbon is the same. Carbon is made from different things and can be activated differently.
Cheap gac (granulated activated carbon) can leech ammonia into the water. Randy holmes Farley did several comparisons pitting different carbons against each other.
Ideally you want to have slow flow in a reactor. Im lazy and found just putting carbon in a mesh bag works good enough. And replacing it only takes 2-3 mins. Putting it into the filter sock where the overflow water enters the sump just causes the carbon to break up. Leaving small carbon pieces everywhere.