if you're running a FOWLR
w a DSB
then you don't really need the pads at all. They are more likely to trap detritus and raise nitrate levels unless you're cleaning them constantly (every couple of days IMO
). Your LR
will handle all of the biological filtration you need.
The carbon, as well, most folks only run occasionaly. If you want to use it all the time, I would say replace it once every week or two.