Algae is caused by more light, not less. It's possible that the lighting you have, plus the indirect sunlight, is too much for the tank (as in, you don't have CO2, so there's a finite limit to how much light the tank can have before algae starts growing). Possibly the sun shifting through the year is now causing it to shine more directly on your tank? I'd reduce the lighting to 8 hours or so, and maybe try dosing with Flourish Excel (which is theoretically CO2 in liquid form - though really it's a poor substitute).
Getting rid of algae is one issue (check the algae forum for the excel thread). Keeping it from returning is a completely different issue.
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