Is aquarium algae harmful or just unsightly?

They are just unsightly. You can try to get rid of the algae by getting a couple of algae eaters (otocinclus or plecos, but if plecos just get one) so you don't have to continually clean the algae off. A way to prevent algae growth in the first place is to keep your tank out of direct sunlight, and if that can't be helped, get an aquarium background and cover the back and sides of the tank, lessening the amount of sun the tank gets. You can also reduce the amount of time your tank lights are on. When I keep mine on all day, I have serious problems with algae, so just keep yours on for like 4-6 hours a day. Hope this helped!  more