Here's some interesting stuff that was posted on PFK, some sort of silicon dioxide molecules that coat the glass and supposedly repels algae. It'll be interesting to see how well it works once people start using it.
Could you clean the aquarium's surface with a product like Bon-Ami and then apply Rain-X or DuPont's similar product?
From the MSDS on Rain X The Invisible Windshield Wiper...
Its active ingredients are Silicone Oils, Siloxanes and Silicones, di-Me, hydroxy-terminated. These chemicals form Si-O-Si bonds with the surface of the windshield and have Methyl-terminated hydrophobic tail group which repels water. So, it should only work on true glass that is silica based, as the bond will only form with the Si-OH groups that are on the surface of the glass.
L-Si-O-Si-O-CH2-CH2-CH3 ---> Water
A-Si-O-Si-O-CH2-CH2-CH3 ---> Water
S-Si-O-Si-O-CH2-CH2-CH3 ---> Water