Tank smells powerfully of rotten eggs

[tentacles]

I don't think carbon will adsorb H2S under normal conditions; Wikipedia mentions using carbon as a substrate coated with a catalyst for reacting the H2S into less toxic/smelly compounds.

I think your best bet is probably some light peroxide dosing. And maybe check that biopellet reactor for cleanliness, if it's clogged and that smell is the bacteria dying off...