Are you testing your water for pH, ammonia, nitrite and nitrate?
I don't use a cannister but there is a lot of annecdotal evidence that they are 'nitrate factories'. That problem can probably be elminated by removing and cleaning the filter pads every week. The carbon pad sounds like it doesn;t do much at all. You can probably get rid of it. I beleive that filter has 3 compartments for media. You can run a good quality GAC
in one of those for a week, then remove it.
Hopefully someone who runs a similar set up will chime in.
How long has your tank been set up?