Green Algae Problem

[one_eyed_wonder]

need some help please..

one of my tanks that has been running for 18 months is all of a sudden covered in green algae! I have tried to remove but after a day or two it comes back! it is everywhere, can hardly see the sand! the tank has been fine in the past and it is side by side with another tank that is fine.

and advice on how to control would be greatly appreciated

thanks