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There is an Oil Slick Film Covering My Pond. How Do I Get Rid of It? – Pond & Lake Q & A

Algae, Duckweed, Watermeal & Pollen Identification

Q: There is a brown rust-like film/oil slick covering my pond. Do you have an idea what this might be and how to get rid of it? – Several Customers

A: There are several things this could be: Algae, duckweed, watermeal or pollen.

Determining if Algae is a Problem: Filamentous Algae will float around the pond’s edges in mats while Planktonic Algae will make the whole body of water to look like a “pea soup” green color. If this is the case, using Algae Defense® Algaecide will provide quick control. Follow up with PondClear™ for long-term clear water.

Determining if Duckweed or Watermeal is the Problem: Duckweed and Watermeal are very rapid growers and will cover an entire pond if they get out of control. Looking to the pictures to the left, you can see that Duckweed is a small plant the size of a pencil eraser, while Watermeal is about the size of the tip of a pencil. If you determine that you have Duckweed or Watermeal. Your only long-term option is Sonar™ A.S. for Duckweed and Clipper™ for Watermeal. If you only require short-term control (3-4 weeks) for an event or party Ultra PondWeed Defense®” may be used.

Determining if Pollen is the Problem: What may look like a greenish, brownish algae, may actually be pollen. Pollen may even cause an oil-slick or film on the surface of the pond. There is no magic product that will give you a quick fix. Many times a heavy rain will settle it to the bottom. In many cases if your pond receives good circulation from an aerator or fountain you will not see pollen becoming much of a problem.

20 Responses

  1. Is it possible that this could happen on just dirt – a place that receives a lot of water run off (from playground to mulched area)?

    • Hi Dennis – Dirt entering the water from runoff usually mixes into the water and causes the water itself to look cloudy. The only way I would see dirt having the “oil slick” effect would be if it was a really dry day and dust was able to land on the pond’s surface but it would still be a noticeable difference between dirt particles on the water surface and an oily look from pollen.

      • I wish I could show you a picture. It isn’t a pond – but our landscaper said the oily green stuff we have on our playground is similar to the stuff on a pond.

        It is on dirt next to pavement. Is that possible?

      • Hi Dennis – I’m not really sure about this. If there was standing water then maybe there would be algae or something on this water. If you have a picture you can email it to us at pondhelp@thepondguy .com

  2. My oil scum looks to be from pellet koi food. The bubbles from the aerators make it look even worse! Unfortunately I only have a net as a skimmer, and not a proper installed skimmer. I have a black straight-lined pond with no plants. How can I best remove the scum?

    • Hi Carolyn – We recommend only feeding fish what they can eat in 5 minutes, any leftover food should be removed with a net. As for the scum, is it floating? Can it be caught in net? or is it more like a slick?

  3. Hi, there are oil wells right next to our land and the owner just drilled back to back on our land, he was sure he’d strike again and said he’d be knocking on our door if he struck. I don’t believe he did though at least not in that location. we have mineral rock, big rocks and oil looking deposits under our natural pond and other areas, I really believe we have oil but can’t afford the expensive test and drills. what can we do to maybe find out. Thanks for any suggestions. Fran

  4. We put a threw a handful of buttered popcorn i into a lake and it caused the pollen to almost magically move away from shore to the center of the lake leaving a wonderfully clear surface in its wake. What caused that??

  5. i have a half acre pond and i would like to know how to get rid of the pollen i think one problem i have is that it never overflows the water seeps under the spill way but it stays full any suggestions

  6. I have a oil slick covering parts of my one year old, 1 acre pond. It blows around into windy areas. I also have a brownish scum like substance that gathers on the edges. Last year I tried using dawn detergent in a ortha lawn hose dispenser, that seem to break it up. My pond has been completely full for 6 months. My fish seem to be unaffected. No problems in the past winter as the pond stayed frozen most of that period. I first noticed it this year when the pollen started falling. I feed my fish pelleted fish food, but rarely see any food remain uneaten. What do you suggest to remedy my problem

    • Hi Brian,

      When most customers are describing an “oil slick” look to their pond is usually is caused by the pollen. This won’t affect your pond or the fish and should break up on its own with a few good rains or once the pollen calms down. Using surfactants to break the water tension will help to alleviate the problem however I would not reccomend using dish detergent in your pond. There are aquatic safe surfactants that you could use or just simply a garden hose that will break up the debris. The brown debris on the edges of the pond is most likely algae or dead algae. You can rake this out or use aeration and natural bacteria to help break it down.

  7. I have had an oil slick issue in my pond for 2 weeks now. I have a waterfall set up and use the smaller pond at the top of the waterfall for my filtration system. My husband added more rocks to the top pond to help prevent the filter material from floating, he rinsed the rocks first (so he says) and all of a sudden my pond turns almost black and there was a very visible OIL SLICK completely covering my pond. I agree that it is mineral related. My fish were unaffected by this drastic change to their environment. I however went crazy trying to find and fix the issue. Finally I did a 100% water change, I actually put my fish in fresh water in a 55 gallon can, bilged out all of my pond, used dawn dish soap to wash the whole pond, liner, waterfall and all the large decorative rocks, I attempted to clean the new rocks but was unable to completely rid them of the slick feel. I even dipped my fish twice in 2 different tubs of water to remove any residual oil that may be on them. After 6 hours of draining, scrubbing, rinsing, and filling my 1100 gallon pond my hard work paid off! My pond looked great until I submersed the pump ( the pump was the only item I forgot to scrub and wipe down) There was enough oil residue left on the housing of the pump to create a very small hardly noticeable oil slick, even if noone else sees it I know its there. I am not as worried now since I know my fish arent bothered by it a bit.

    • All you have to do is skim the slick with a strainer covered with a thin kitchen towel or a paper towel, (or us a pool skimmer, covered with a towel if your pond is big) then let the trapped water leak through, do this a few times, then replace the napkin or rinse towel and go again. I had some oil–that also really annoyed me–on my small pond, but got rid of it by skimming with this method. This idea just popped into my head while I was racking my brain about how to get rid of the oil.

  8. I have been feeding my fish a trout foodstuff. This smells like it has cod liver oil in it. Thanks for the pollen idea. I feel it could be that as well. Just to say to your readers- watch what you feed your fish on!


  9. We have the same “oil slick” kind of problem on our pond as well and it definetly isn’t pollen. We have several natural springs that it come out of the ground with it. We had a man with the state of Tx come out to look and see what it is and he says that it is very normal part of the water around here and it isn’t any kind of sewage or anything like that. I think it must be the “mineral” almost like oil coming out of the ground. Who knows? I do wish we could do something about it.

  10. Hey Angela,

    Could you e-mail me a picture of your pond with that oil slick on it? From how you describe it, it sounds very familiar to pollen. Especially since it re-appears in the summer months, moves with the wind and the fact that is disappears after it rains. Also, when trying to skim pollen with a skimmer net, it is almost as if you didn’t pick up anything. E-mail me some pictures and I will get back to you on it. My E-mail is: joe@thepondguy.com

  11. I live in north Georgia and also have the “oil slick” problem with my pond. All of the pictures and descriptions in your response do not appear to apply to this problem. The “oil slick” will move and gather as the wind blows it. It disapears after a rain, but returns in the next day or so. It is not noticeable during the winter monts, but re-appears every summer. We have even tried to “skim” the pond to remove it. I have been told several things over the years from it comes from minerals in the water to it is created by decomposing matter. Do you have any further suggestions?

    • Angela…
      I picked this up from an environmental agency that does work in the great lakes area:
      ” have you ever seen a stream, wetland, pond or ditch with a rust-colored substance all over the bottom and sides? Commonly, if you see this substance you may also see an oily film on the surface of the water. Although this looks like pollution, it is actually not. In areas that have iron-rich water, ground water seepage, and low flow, naturally occurring bacteria called Iron Bacteria oxidize iron for energy. The by-product is ferric iron, which becomes iron oxide when it is exposed to air and water. Iron Bacteria are not known to pose any environmental or human health risk. ”

      Hope this is helpful…I have the same occurence on my newly build 1 acre pond in Missouri.
      Jamie Ferrugia

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