When this happens to my Glosso it usually means that it is time to change bulbs.
But it could be an iron deficiency too. I routinely over dose on the Flourish iron - a bit. And I also have Flourite as a substrate. So in your case try dosing iron. Another thing my Glosso really likes is phosphate. It responds rather quickly when I up the phosphates.
The fixture is nearly brand new. I'm pretty sure the light that came with it was new (it was a refurbished unit) but either way, I know one bulb is new
. They're dual daylight 6700k/10000k Sunpaq bulbs. With over 4 wpg, hopefully light isn't my issue - maybe too much light.
It could also be phosphate. I've throttled down phosphate dosing too to keep levels hovering at 1 ppm, but I think phosphates may be one of those things that you can't do any damage with if you have too much (within reason).
I'll try upping the iron dosage as well as the phosphate, maybe a little bit of everything. There's no fish in the tank though so I might need to add some more fertilizer to compensate. There will be angels soon though. Mwahaha.