I would try plants, first and foremost,
to compete for the nutrients that that algae
is using to grow.
Fish will only add more nutrients and would
probably escape to the sewer system, or their infants
would. Weeeee, freedom! ;-)

I'd keep the plants in pots so they don't overgrow
the system and start plugging it up.
Cattails, pickerel, arrowhead, lizard's tail, water
iris are a few that come to mind. Nice, tough
marginal plants, can grow with two to about
six inches of water over their crowns.
If you have any pictures you could post on a web
page we could probably even refine our advice
further.
It sounds like an interesting and unique problem!

k :-)
http://tinyurl.com/6bguh ~ new pond keeper info
http://tinyurl.com/yp64db ~ slide show of pond