We left ours on at the end of the summer for about 4 days in a row, no one was swimming and the weather was overcast. Didnt get any blooms but noticed that my Chlorine level kept going up and up (SWG set at 40%). The Pool store said I should keep the cover off during the day to prevent this especially if no one is swimming, they also mentioned something about having the cover on could cause algae to start. I suspect I didnt get any blooms due to a south facing pool and my pump was running 24/7 at the time. I guess even water needs to breathe.