Nope. You'd have more iron coming out of the gfo itself than the plastic in the reactor. I mean, don't use the water, but the reactors fine. Especially since you're running carbon in it. Rust is very stainy, I used a binder clip to hold a filter sock in place a long time ago and it's still discolored at that spot despite weekly bleaching. If there was anything dangerous to come out of gfo we'd know by now