Thread algae frustration

Well I awoke this morn to a 75 gallon tank completely filled with thread algae. Spent 4 hrs cleaning all the gunk out today. I couldn't see any fish or plants in there. I'm using the E.I. dosing method and You cant count the bubbles per second I'm putting in this tank. I understand 95% of all algae issues are Co2 related but what about the other 5%. What else causes thread algae. I can literally watch it grow in minutes.

Tank specs
75 gallon
4x55 pc lighting
pressurized Co2
Rena filstar xp3 filter
External Co2 reactor (Thanks Matt)
Soilmaster/3M colorquartz substrate.
No3 ~15ppm
Po4 ?
Ph 6.8
kh 6-7