We know eachother enough to rule out such thinghs, CO2 is really all that is left here. TMG is a pretty good deal Greg might carry it if I hear back from him.5 liter jugs are decent deal.
That will rule out most things.

From there, adding a carbon filter to the tap would be the next thing.
With that much light and ferts, the CO2 demand is really quite high.

I have a client's tank with 4x 250w + 8x 55w PC's and add about 10-14 large bubbls/sec. That's light CO2 also

Regards,
Tom Barr