You are probably right. I took it off after 5 days and the Red Macro-algae I am trying to kill only a slight die back. I know some people have used plastic to kill green hair algae in only 3 days but this red stuff is hard to kill.
I used black plastic from a garbage and folded it double bag but I think some light goes right thru it. I'm going to try something that will completely block out the light for 5days.