I have a 125 gallon too, Orlando from GLA helped me out quite a bit getting mine dialed in. I was using a single diffuser hooked up to the outlet tube, flowing from the corner of the tank. I had a feeling I wasn't getting even distribution, testing the PH and KH method proved that.
First he had me remove the spray bar (I was using the Eheim spray bar). The back pressure of the spray bar limits the circulation, using an open nozzle created the right kind of turbulence to circulate the water. He used more scientific words to describe it.
Second, he had me make sure there were very little to no rippling on the surface. I had worked under the impression that a small ripple was good, yet he asked me to make sure there was essentially none. So I pointed my circulation pumps downward, and to ease my mind of gas exchange I started running an airstone a couple hours over night. I had zero movement at the surface.
Last, since I was running two Eheims at the time, I moved the outlets to the middle of the tank, pointed them towards each other so when the water comes out, one outlet containing the co2 the other without, the flow mixes together and provided a more evenly distribution of co2 across the 6' tank.
So those three things solved the issue, after that it was just dialing the appropriate amount of gas into the tank, co2 concentration is balanced, pearling across the tank the last few hours of the night, growth has been great, and once I got it dialed in my algae issues were greatly reduced. All from a single inline diffuser on just one of the filters.
With the power of an FX6 I would imagine you could have the co2 enter from one corner of the tank and it would get pulled across to balance. Worth a try taking the spray bar off and let it flow out of the nozzle with less obstruction.