Well, a few pointers:
If the conditions in the tank are not altered then the algae will keep coming back.
Look into the light, fertilizer and CO2 balance. With direct sun the tank will need high tech levels of fertilizer and pressurized CO2.
Correct the above issues over time, and remove the algae by hand. Use several things to gauge the progress of those changes: Health of the fish, growth of the plants, minimal regrowth of the algae.
Once you have things stable, and the algae pretty much is not growing, you could hit it with something like Excel or Hydrogen Peroxide to finish knocking it out. You could do this without making the changes, but it will just come back.