It only affects certain algae, black brush for instance. And it doesn't immediately wipe it out. It has an impact, but it's not a comprehensive algaecide. Algae eaters - with the exception of the SAE - don't eat black brush algae anyway. I wouldn't be too worried about your algae eater starving as a result of dosing excel. A lot of algae eaters also graze on biofilms that bacteria form.