Looks good....just a thought, though....5 glo fish are gonna be lost in a 125. I would either up the numbers or ditch them all together. Plus, they are pretty expensive IMO. I think you could easily up them to 10-12 and still be fine. Not that much difference for a 125.
Sorry I can't help with the FX5, never had one. Only suggestion is you don't really need carbon. I don't think most people use it. I only use it to remove meds.
Since everyone has already answered about the stock, I was just curious, you said "The other two tanks I have are a 55 gallon tank with a small Oscar" Where is the oscar going? You don't plan to keep him in the 55, do you? He'd be better off in the 120 gallon by himself or with a few tank mates, but the 55 is far too small for him, I'm sure as you know, they grow QUICK!