I am running 2 FX5s on a 120g tank and I want to know if it would be ok to swap out all the media in one of them. I bought pond matrix as the biomax is breaking down badly in one FX5.
Since your running 2 fx5's I'd just replace the one you want the stuff gone. I run two on mine and have replaced the media right out leaving the other untouched. Worked out fine but that's just me.