Pond help, please : algae
 

I have a rather unique pond. It's actually an underground stream that once
made my back yard a swamp. I have corralled it somewhat and turned it into a
nice little ( 1000 gallon ) pond. Water enters the pond under it's own
power (through a pipe leading into a scrap fire hydrant ). It drains (
through a pipe and eventually into the storm sewer ) through a lowered part
of the pond wall. There is no filtration. Although I always keep sufficient
gold fish to keep the resident Heron and Racoon populations happy and fat,
it is primarily a plant pond. My trouble is what I believe is called string
algae. I have been told that because the water comes from deep spring it is
cold, but very rich in minerals ( I have had it tested of course ). Plants
grow like crazy - including the string algae. Because I have an open system
I don't think chemicals to lower the pH levels ( I believe that's the usual
treatment ) would work. As it is, I simply rake them off the rocks and
compost them - but in summer if I forget for a few days the pond gets choked
with the stuff. I have a small waterfall contraption I built that requires
a pump - but the pump gets choked in no time at all. Any thoughts or idea ?