When I set up my nano tank, it came with a skimmer that had a little pad of carbon in the white felt in a plastic frame (like the disposable filter cartridge) I didn’t want to use that skimmer but decided to toss the carbon pad in the sump, it’s under loads of alfagrog now but after reading effects long term I’m unsure whether to dig it out or not, it’s about an inch long by about 2 inches wide and about 2cm thick