No tank is algae free, nor should it be.
The CO2 in any form helps the plants out compete the algae. In the case of Excel, it actually kills some kinds of algae, like black brush algae.
Almost all planted tank owners also employ algae eating critters like algae eating snails, fish and shrimp. Even the master of planted, Takashi Amano, uses algae eaters.
Currently, some of my best critter based algae control comes from the lowly pond snails that came in on my plants. I actually love them for the job they do.
I have alway used Excel in my low-medium light, low tech tank. I just added DIY CO2 with Excel. I believe that CO2 can benefit any planted tank. It helps with but does not eliminate algae. It definitely and dramatically promotes plant growth. Without it, algae can run rampant as it did when I got lax with my Excel dosing last August.