Have you tried using a Blue Dye?

At this stage I have to declare an interest in that my Company sells a couple of products that will "reduce" the growth of Algae.

The first product, DyoFix Pond Blue works by reflecting the sun's natural Ultra Violet light so that it does not reach the bottom of the pond, and without UV the algae has nothing to feed on. The product is safe for fish and any other creatures that live in, or visit ponds. Do not be alarmed at the thought of having a bright blue pond as, at the correct dosage, the shade is almost imperceptable (I hope that is spelt correctly!)

The Blue dye (at correct dosage) only works with ponds that are at least 24 inches deep

The other product is for smaller, fish free ponds, DyoFix Pond Black. You may have seen it in use on the BBC show "A Year at Kew". This is also harmless, but dyes the water Black meaning no light gets through at all, but the reflective properties are fantastic and it makes the pond look like it is a mile deep!!

Examples at www.dyofix.co.uk