I've never used GFO. I can't help much on the subject of reactors since I don't have any and never have, partly because I don't do sumps either. I use phosphate removing pads in one tank (the Al-based ones), but it's a kind of minimal effort at phosphate removal compared to something like GFO. I mainly rely on biological exporting for nitrate and phosphate. Tcamos is definitely the better one to go to for the topic of GFO and reactors.