They're fine if you don't use the sponges or mind cleaning them every 2-3 days religiously if you do, other wise they become a nitrate factory. Also, if the flow isn't high enough to fluidize the gfo you will go through it much quicker since it will clump. This subject comes up from time to time and the bottom line is, yes, it can work, but separate reactors for GFO and carbon will give you the best bang for your buck by optimizing the useful life of the media.