Why Your Pond Has a Film on Top and What it Means

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Ponds can be beautiful fixtures in your garden or backyard, but sometimes even the ponds of the most devoted homeowners can end up developing a nasty-looking film.

Most pond films are made of biofilm, a mix of dead algae or protozoa that occurs when there is an imbalance of nutrients in the water. Live algae blooms or pollen from nearby plants can also cause similar films. Although ugly, these films are usually harmless.

There are a million reasons your pond could have developed this awful film. Let’s take a closer look at each kind so we can figure out what you’ve got growing in your pond


Biofilms usually appear as an iridescent oily coating over the top of your pond. Because of this, they are often mistaken for oil slicks. However, unlike oil slicks, biofilms can occur in any pond regardless of their proximity to oil.

Biofilms occur when the amount of biological matter in the water is more than the algae or other bacteria in the water can decompose. The reasons why this may happen are not very well understood, but they’re a couple of environments in which it is more common than others. They are composed mostly of dead algae and protozoa as well as some non-decomposed organic matter.

Biofilms are especially common in water that has a lower PH (meaning that it is a little bit acidic.) They also tend to be worse in warmer parts of the year, especially when the temperature gets to be higher than 70 Fahrenheit.

The effects of rainfall on biofilms also seems to be fairly unpredictable.

Biofilms can be somewhat difficult to remove from a pond due to their mysterious nature, but one thing that can potentially fix the problem is raising the PH of your pond using something like lime or soda ash.

Adding some kinds of beneficial bacteria can also help the problem. Aeration can also be useful.

One thing that can make biofilms worse is the repeated use of algicides or other chemicals. If you have a biofilm problem, you should avoid using any of these chemicals.

If you’re having trouble telling if the film is a biofilm or an oil slick, you can do an easy test with a stick. Pull the stick through the film and watch what it does.

An oil slick will swirl around in the wake of the stick, while a biofilm will break into smaller pieces.

Algae Bloom

There are two different kinds of an algae bloom that are common in domestic ponds: Planktonic and Filamentous. Both are harmless in small amounts and are normal to see at any time during the spring.

Planktonic Algae

Planktonic algae look kind of like yellow or green paint on the surface of the water. It will usually bloom in warmer parts of the year and die off in the winter.

Planktonic algae are ubiquitous present in water, and it is mostly harmless to humans. It serves as the base of any aquatic food chain, so if you have fish you have an interest in keeping this stuff around.

However, an excess of this algae can be harmful to fish when it dies, as it will all decompose at the same time, resulting in all the oxygen being drained from the water.

A common test for checking whether you have too many algae is to take a small object that will sink, tie it to a string that is longer than 18 inches, mark the 18-inch spot on the string, and then lower the string into the water until it hits that mark.

If you can still see the object, then you’re probably good. If not, you may want to attempt some control measures.

Aeration can help control an overly large population of planktonic algae, but the best way is really just to decrease the number of nutrients in the water for the planktonic to grow off of.

Mostly, this includes preventing animals from defecating in or near the pond and preventing fertilizer from your lawn or garden from running off into the pond.

You can use chemical agents to kill these algae, but it usually isn’t a good idea since the algae will just grow back as soon as the chemicals are gone. Mechanical means will also fail to remove this kind of algae, as it is just too small

Filamentous Algae

Filamentous algae is much closer in structure to a plant than planktonic algae is. Because of this it tends to appear as a blue or green mat on top of the water’s surface.

This kind of algae is also usually harmless to the pond, although it’s important to note that it tends to be chock-full of bacteria and other matter that can be toxic to humans, so it’s generally a bad idea to touch it.

A healthy population of filamentous algae will look like a series of small mats on the water, maybe collecting along the side of the pond or around a particular rock.

While it is usually harmless, it can potentially grow enough to cover the entire pond and block out all light beneath it and take up all the pond’s oxygen, which can be harmful to any animals living down there. Unless the water has an excess of nutrients, however, this is uncommon. If it does happen to occur, there are a few things that you can do to deal with the problem without turning to herbicides.

Any of the measures that can deal with planktonic algae will also work for filamentous algae, although there are also a few methods that are unique for fixing this particular problem.

One method is to introduce a population of Blue Tilapia, a kind of fish that loves to eat filamentous algae. This won’t completely solve the problem though, and you’ll still need to find a way to decrease the nutritional value of the water. Blue Tilapia is a very expensive solution, as it is a tropical fish and will die as soon as the winter comes.

You can also scoop the algae out of the water by hand or with a machine as long as you’re careful. However, this will only temporarily solve the problem, as like planktonic algae it will tend to grow back over time.


Pollen can also collect on the surface of the water during springtime. again, this is completely harmless, but it can look a little ugly if there’s enough of it. Pollen will stick to the surface of your pond because of surface tension. If you’ve ever seen a water skimmer bug, you know all about surface tension.

It is so small and light that it can’t break through the surface of the water, so even though it tends to sink once it gets under the surface, it will float until something is done about it.

The easiest thing to do is to just wait for it to rain. Rain will break the surface tension of the water, allowing the pollen to get below the surface and to escape through whatever outtake you have.

You can simulate this with a garden hose, or by installing something like a waterfall or a fountain to provide a bit of aeration to the pond. Anything that keeps the surface of the water moving will be effective in fixing the problem of pollen.


Aeration can help in almost all of these situations, but how can you, the reader, provide aeration to your pond? And what exactly is aeration?

Essentially, aeration is any method by which oxygen is added into the water. This can be through a subsurface device or a device on the surface.

Surface aeration works by mixing air and water together, giving the water a chance to steal just a little bit of that sweet sweet oxygen. In a small garden pond, a fountain or waterfall can do this job just fine.

Subsurface aeration works by releasing a large number of air bubbles into the water from below the surface. These bubbles are usually produced using compressed air from the surface and can be produced using anything from jets to an ordinary air pump.

Either kind of aeration can be found on the market (or installed by a specialist) if you feel like you have a need for it. Considering the many helpful side effects that it has, I would feel like it’s usually a safe investment for pond owners that will help keep their ponds nice and scum-free.

Algicides and Herbacides

Chemicals like these seem like an easy fix to the problem of pond film, but in reality, they just aren’t. Any kind of chemicals designed to kill plants or animals can be extremely damaging to your pond’s ecosystem, which exists whether you have fish in it or not.

Just about all chemicals that hurt algae will hurt fish too, and the consequences of hurting either will be a gross biofilm on the top of your pond’s surface. I wouldn’t recommend these kinds of extreme measures except as a last resort.

Perhaps if my pond had developed a culture of dangerous algae like blue-green cyanobacteria I would consider dealing with the toxin-creating algae with chemicals. However, in any normal situation, they’re just as likely to make things worse as they are to help.

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We always wanted a fountain of some kind at our house, but professional installation was just too pricey. So, we decided to make our own little fountain and after learning how, we thought we should share our experiences to help people in our same situation.

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