Not to jump on the bandwagon here, but the most likely cause for your algae is phosphates. I know you may feel that you don't have any. In fact, you may be able to test with many different test kits and still get a reading of 0. The reason is that all you phosphates are getting used by the algae. If you try manually harvesting as much algae as possible, wait about 12 hours, then test - you'll probably see the phosphates.
Just some food for thought.
GL
hooked