Green Pond Water

Our little ponds are covered in some green yuck. I think it is string algae, but I'm not sure. The ponds are fed by a spring whose Entrance Into The Light Of Day is hidden. A crack in the large rock wall, perhaps? We were told that it runs at about 25 gallons per minute, but I have trouble believing that. At that rate of flow, I'm confused as to why the water is still enough for algae growth. Anyway, we want to harvest the algae for compost. Now, how to get it?

 This thing is a pond rake. It has a removable Styrofoam float to keep it on the surface of the water. The float can be removed if you want to drag the bottom of the pond. I have no idea what lives on the bottom of our pond, as we've never been able to see through the water. I like the rake pictured below because it has a cool name and a cool shape.

I don't know if it would work very well - I just like its sharp, pointy teeth. And even though these are small ponds, you can't just reach across the surface and get it all. I can't see buying a little boat - even a buddy boat - to get it all, so there is more thunking to be done.