Go to a pet store and buy an algae pad that says it is made for plastic aquariums. Petsmart carries them if there are no local independent petshops. I have used them for years with no scratches. They are softer than the pads used on regular glass tanks or dishes.

i use a regular soft sponge. You have to rub hard but the alge comes off without scratches.

My dad taught me to use a razor blade and just scrape it off. Works well for glass and plastic.

Also, bio bugs (bacterial cultures) will help to balance the water chemistry and algae will be less.

(staff at this site - PHD - explained this to me in detail!)

Here is a link that might be useful: Bio Future

a pleco will do the job effectively. I have a kind which remains the size of up to 7 inches. Chinese algae eaters are effective in cleaning algae off leaves and rockery, but not the glass.