Get a butt-load of mexican turbo snails, add a phosban reactor, decreased the amount off feeding, increase water changes. Don't believe your phosphate testkit.
Truer words were never spoken
You should change the GFO weekly and try Phosban rather than what you are using. Changing it every 3 weeks when you have an issue like yours is nit near often enough. That and the increased water changes are probably the most critical.
Sorry for all of the questions but your answers are helpful...
What are the dimensions of the tank?
You say you are running 12 T5's? Can I assume they are 2 rows of 6 bulbs, or staggered in the canopy?
That's a LOTTA light...best to cut it back by a couple of hours a day at this point.
Just exactly when did you add them (you said they are brand new)...?
Did you add the fuge at the same time?
..and in case someone volunteers to drop by..what town do you live in?
btw - the air bubbles are carbon dioxide - since algae is a plant, it's taking in oxygen and giving off the carbon dioxide - the air bubbles you see. They will increase during the day as long as you have the algae issue going on..not to worry about them