not neccessarily. i run a skimmer but i feed ALOT therefore having excess nitrates. that is what causes most algae. ALSO can be part of the cause for cyano bacteria (commonly reffered to as red slime algae, but it is not algae) If you have good flow, do not over feed and have good cleaning practices (and 2 out of 3 DOES NOT count) then you will normally not have a problem. Having a skimmer HELPS but is not the end all be all.