An interesting discussion on algae, nutrients, balance and the like. Build your own Algae Scraper Potassium Permangenate , a nasty oxydizing chemical sometimes used as a remedy. Bleach is a more effective means, though it can also destroy your plants in the process. Simazine is a herbicide which can be used as a last resort. Barley Straw is cited as a natural algae preventative.
Debate rages on as to why.
However, this may cause trouble with the fish's gills.