I just set up a 2 reactor system. Used 2 150 TLF phosban reactors, one for carbon and one for bio pellets.
I used GFO
for a few days & got my phosphate from .5 to almost 0 then removed the GFO
and replaced with bio pellets.
Bio pellets are a form of carbon dosing, where the pellets break down and release carbon (not the same as activated carbon) to develop bacteria that consume nitrate & phosphate and are subsequently removed by the skimmer.
I have the reactors in line with my Fluval 405, and seems to be working great. No extra pumps or lines to the tank.