this is what I do...everyone has a different opinion so im just giving you mine...
I have a few tanks (no mention on how many LOL)
on my overstocked 75 gal, I run carbon to help with the nitrates and
ammonia..its changed out every 3 weeks and never a day longer...so it works awesome..but on my smaller tanks that arent over stocked, I cut a hole in the bag part and dump the carbon out before using it...
carbon use is mainly bad when not changed out every 3 weeks or so..because it only absorbs so much then the rest goes back to the tank..so as long as your changing it regularly, its fine IMO ...