Greasy film on the surface of the aquarium: causes and natural solutions
F. MattierThis is a great classic of aquarium keeping. One morning, you turn on the light in your tank, lean over it, and suddenly, panic sets in. A whitish, iridescent layer is stagnant on the surface. Light penetrates less effectively, bubbles have trouble bursting, and you get the unpleasant feeling of dirty water, as if a drop of motor oil had fallen into it.
Many of you write to me, somewhat worried, when this happens. Often, the immediate reaction is to rush out and buy technological gadgets to remove this film. Yet the appearance of a greasy film on the surface of the aquarium is not an unavoidable technological problem: it is simply a biological message that your ecosystem is trying to send you.
Today, I will explain how to decipher this message and how to entrust the cleaning to genuine natural helpers.
The visual observation and the myth of "introduction"
When we observe this oily film on the aquarium water, we often feel that we have made a mistake. We think: "I must have put dirty hands in the water" or "I introduced a bacterium with my latest plant."
It is time to dispel a major myth, exactly the same one surrounding the appearance of algae. You did not "introduce" anything at all! What you are observing is an aquarium bacterial film (or biofilm). The bacteria that make it up (like algae, for that matter) are present everywhere and all the time: in the water, in the air in your living room, and in dust.
They are opportunistic. As long as the environment is balanced, they remain invisible. But if the conditions you provide suddenly become favorable, they multiply exponentially until they form this gelatinous film, visible to the naked eye. So this is not an outside invasion; it is an internal imbalance.
The emergency trick: using a paper towel
Before talking about biology, I know that this iridescent film is very unsightly and blocks gas exchange at the surface. There is a remarkably effective DIY emergency technique for removing surface biofilm in just a few seconds.
Take a simple sheet of regular paper towel (without fragrance or colored patterns). Lay it completely flat directly on the surface of your aquarium. Let it soak for two to three seconds, no longer, then gently remove it by one corner. Magic: the bacterial film will stick entirely to the paper!
However, I must clarify one fundamental point: this trick is purely aesthetic and temporary. It does absolutely nothing to address the underlying cause of the problem. If you settle for this alone, the film will return within forty-eight hours.
The technological illusion: the skimmer versus the small air stone
Faced with this recurring problem, the aquarium industry invented a very popular gadget: the skimmer. For beginners, it is a small motorized surface filter that draws water in from above, takes in the greasy film, and expels clean water.
The idea seems appealing, but installing a skimmer in a natural aquarium is an ecological absurdity. As is often the case with conventional technology, this device merely sweeps the symptom under the rug without ever treating the disease. Worse still, in a natural tank, its powerful suction is a constant danger: it will inevitably swallow your lovely little floating plants and fatally suck in fry or young shrimp that live precisely near the surface.
The gentle alternative: if you really want to mechanically break up this film while your biological balance is restored, simply use a small air stone (an air pump) set to a very gentle flow. The simple rippling caused by bubbles bursting at the surface will continuously break up the biofilm, promoting water oxygenation without any risk of suction for your plants or animals.
The real cause: excess organic matter
If we switch off the skimmer and throw away the paper towel, we need to ask the real question: what is feeding these bacteria?
The answer is almost always the same: it is you, or rather, your slightly too-heavy hand when feeding. Biofilm appears in large amounts because of overfeeding. Excess protein, uneaten pellet remains, and the abundant waste that results create a true feast in the water column. The bacteria then rush to the surface, where oxygen is most abundant, to digest this organic soup.
So the very first action to take is not to pull out your credit card to buy a motorized device, but simply to halve your food portions for a week or two.
The biological heroes: bladder snails and Floating frogbit
In Aquazolla's low-tech philosophy, every problem finds its solution in nature. Against surface film, we have two formidable helpers.
Bladder snails, specialists in acrobatic work
If you observe these increasingly popular little aquarium snails closely, you will notice fascinating behavior. Bladder snails are absolute specialists in biofilm. This very greasy film is what attracts them. They swim upside down, with their shells hanging toward the bottom, literally stuck to the surface, feeding on it in an endless ballet.
That is the secret: by feeding your fish less, you will slightly starve your bladder snails (which until then had been content with the leftovers that fell to the bottom). When hungry, they will logically rise to the surface and meticulously devour the entire greasy film.
👉 Adopt the best surface-cleaning team with our bladder snail colonies.
Floating frogbit (Limnobium laevigatum) to cut off their food supply
The second step is to deprive the bacteria of their food at the source. To absorb excess nutrients present in the water (nitrates, phosphates, and dissolved waste), nothing surpasses the absorption speed of floating plants.
I particularly recommend the plant we commonly call Floating frogbit, more precisely Limnobium laevigatum. With its fan-like roots and vigorous growth, it is a formidable plant. As it spreads, it captures organic matter much faster than bacteria, thereby competing with the bacterial film until it disappears on its own, while also keeping your fish safe.
👉 Cut off the bacteria's food supply with the voracious appetite of our Floating frogbit.
Conclusion: accept a living environment
You have to accept that a natural aquarium is a broth of life. Unlike a chlorinated pool, the water in an ecosystem is never sterile. The appearance of a slight biofilm, particularly when starting a tank or after heavily trimming plants, is not serious at all. It is simply a message from your ecosystem signaling a temporary nutrient spike. Listen to it, adjust your feeding, let your snails and plants do their work, and put the motorized vacuums away!
Mattier
👉 Everything you need to know about the aquarium's real cleaners.
FAQ: everything you need to know about aquarium bacterial film
Is bacterial film dangerous for fish?
No, the bacteria that make up this greasy film are absolutely not pathogenic to your fish. The only indirect risk occurs if the film becomes extremely thick (like a crust) and covers 100% of the surface for several days. In this extreme case, it can hinder gas exchange between the air and water, thereby limiting oxygenation in your tank. This is where the paper towel trick proves very useful for immediately restoring the tank's respiration.
Why does the greasy film return after a water change?
This is the biggest frustration for aquarium keepers! We carefully siphon the surface, the water is clear, and the next day the film is back. This is normal: a water change briefly dilutes the pollution, but it does not remove the organic matter accumulated in the substrate, in the plants' natural filter, or the bacteria suspended in the water. As long as nutrients (the cause) are present in excess, the film (the consequence) will re-form in record time.
Do ramshorn snails eat surface film?
ramshorn snails are excellent detritivores that actively contribute to aquarium cleanliness, but they are not great fans of swimming upside down in open water, unlike bladder snails. A ramshorn snail will gladly eat biofilm deposited on glass, roots, or plant leaves, but to remove floating film directly from the surface, bladder snails remain unquestionably the champions of the discipline.