enter image description hereI've recently purchased a property with a pond. There is large amount of algae, producing a scum like film on the surface. I've found chemicals that are safe for the fish (blue gill, bass, and catfish are present) that you can apply, but have also found other options such as raking or planting water lilies. I have yet to try anything, looking for a starting point and most importantly I don't want to harm the fish. Also, considering an aerator.

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    What sort of algae? - many species of catfish and likely the others thrive in eutrophic waters. A picture might help. Have you tried aeration?
    – bob1
    Commented Aug 16, 2021 at 21:25
  • I'm unsure the type, picture added.
    – ejjml428
    Commented Aug 17, 2021 at 20:36
  • Just a note but even if the chemicals are safe for fish (and it's better to read up on that first, especially on long-term effects) they might be unsafe for other organisms. Raking is effective and fairly simple and is (as far as I know) a fairly established technique. Pretty wide floating rake + rope would work here, for instance.
    – stijn
    Commented Aug 18, 2021 at 13:00
  • I expect someone has been doing some kind or care to the pond because of the lack of growth around the shoreline; normally there are things like cattails ( unless it is pasture and livestock graze to the water edge). So what ever care has been done may account for the algae scum. I think a fish trap would be useful to determine what fish may be present to start with. Commented Aug 20, 2021 at 22:35

2 Answers 2


What are you calling algae ? From the question , it sounds like you don't have a background in ponds. Books or internet are good. There are many floating plants but in many ponds I have seen, I never saw floating algae scum. Green water a couple times ; even had it a few days myself long ago. Generally the chemicals are to make money for shops or for swimming pools. Many fish nibble plants, Koi and catfish downright eat them ; so if you use algaecide, you will likely kill or injure fish. If I had macro-algae in a pond , I would add plants and try to determine the source of nutrients. Higher plants will out compete the algae for nutrients and starve it. Water lilies are good but I think floating plants will get nutrients faster. I have duckweed, water sprite, hornwort, anacharis , water lilies, but there are a great many . Some may be illegal in your location , but things like water lettuce, and hyacinth are easy to pull out of the pond after they soak up nutrients and multiply.

  • Nice size pond ; yes that looks like floating algae. I would add water plants and bog plants at the edge. There are certain carp that are supposed to eat algae faster than Koi or regular carp but I never got more info on them. Commented Aug 17, 2021 at 21:25
  • Floating algal scum is common in hot weather on larger bodies of standing water, especially those exposed to nutrient overload such as fertiliser runoff. It's normally a form of cyanobacteria
    – Separatrix
    Commented Aug 31, 2021 at 7:38

It could be due to too much nutrients (fertiliser). In which case algaecides will not help.

Check the watershed of the pond - where the water comes from. See what grazing animals live nearby. For example, if cows are nearby their manure will always enrich the water to give algae.

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