A small pond about 100 feet across in my yard (located to the right in picture below) drains through the small path into a nearly non-grated sewer drain.

In recent years, more and more algae has been flowing through here possibly due to cattails having largely died off.

The algae sits on top of the water and never seems to drain with the water. The algae is 1-2 inches thick in this photo, with maybe a couple inches of water underneath that flows very slowly.

We've had to scoop this stuff out a few times and it's extremely thick and difficult to clean up. It doesn't take long before more flows through and it fills up again.

Any suggestions for ways to get rid of this stuff or prevent it from coming back?

  • Stop fertilizing, for one thing. Is the sewer drain yours or is it town-owned & maintained? For that matter, could you re-landscape so the pond doesn't drain into anywhere? what's feeding it? Commented Jun 26, 2020 at 15:22
  • Where are you -- do you have freezing winters, for one thing? Commented Jun 26, 2020 at 15:22
  • Yes I'm in Minnesota.
    – Baa497
    Commented Jun 26, 2020 at 16:25
  • We don't fertilize anywhere back here, though the neighbor on the other side of that bridge does. There are actually a few larger ponds connected which all flow through here. They are fed from suburban/residential street drains. We've tried getting the city to help for years but that hasn't worked out very well.
    – Baa497
    Commented Jun 26, 2020 at 16:31

2 Answers 2


That looks like the type of algae that is floating IN the water (as opposed to ON it). That means it might be controllable via aeration, either a redesign to add more falling water to get more oxygen into it, (i.e. a small dam and waterfall) or by adding a small pond pump and sprayer.

Secondary to keeping it under control is to take a look at whether you are causing it to over bloom in the first place. Looking at your yard, you might be over feeding your lawn and plants, so the nutrient runoff is getting into that stream and feeding the algae too.


Koi and goldfish will eat about anything that is green. Are there any fish in the pond?

  • Nope mostly just turtles, geese, and ducks.
    – Baa497
    Commented Jun 26, 2020 at 16:58

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