[Delphinus]

Totally different things that just happen to have the word "carbon" in their names.

Carbon aka charcoal is a porous substance and happens to be really good absorbant of other materials. Once it's done absorbing "stuff" though then the pores are clogged and it cannot absorb any more.

Carbon DOSING is a generic term used to reference things like vodka or vinegar or sugar (or combinations thereof) as a source at molecules that involve carbon to be used as a food source or input for metabolic processes that also happen to consume nitrates and/or phosphates as part of the chemical/biological processes.

Biopellets are intended as a convenient method for carbon dosing because whatever polymer they are made of is carbon based and is also porous - but instead of being good for absorbing stuff, the pores are good for creating large surface area for bacteria to colonize. So bacteria feed on the pellets and in process of doing so consumes nitrate and to a smaller degree phosphates out of the water column. The "tumbling" of the pellets ensures that excess bacteria sloughs off, which is thought to be used by filter feeders but in general really requires mostly to be skimmed out (and they do skim out nicely - skimmate production does increase after starting pellets).

You wouldn't want to run them together in the same reactor as they have different lifespans. Pellets could last you months - carbon (depending on the amount you use per system volume) needs to be changed out every 2 to 4 weeks.