Goldfish without a filter: moving from a bowl to a real natural aquarium
F. MattierSome battles take a lifetime. For more than twenty years, I have fought every day to change attitudes in the aquarium hobby and defend a cause close to my heart: that of the goldfish.
It is a painful observation, but the goldfish is probably the most abused pet in terms of numbers. Sold as a toy or won at fairgrounds, it invariably ends up in a glass bowl. It is genuine torture: this tiny container causes its internal organs to atrophy, deprives it of oxygen, and exposes it to rapidly escalating pollution.
My goal has never been to make beginners feel guilty, as they are often misled by sellers and misconceptions. On the contrary, I am here to support you. I always remind people that the absolute ideal for a goldfish remains a large outdoor pond. That is where it truly thrives. But I also know that not everyone is lucky enough to have a garden.
If you live in an apartment and are looking for a way to replace your goldfish's bowl with a genuine ecosystem that respects its needs, you are in the right place. Today, we will see how to design a natural aquarium for a goldfish, lush and self-sustaining, without burdening yourself with noisy and ineffective mechanical technology.
The illusion of the mechanical filter when it comes to nitrates
When deciding to give your pet a larger volume of water, the first instinct is often to buy a huge motorized filter, thinking it will guarantee clean water. This is a widespread commercial illusion.
The role of the aquarium filter needs to be demystified: no mechanical or biological filter sold commercially makes the nitrates produced by the goldfish (NO3) disappear. This fish is a heavy eater and a heavy polluter. Its abundant waste produces ammonia, which is highly toxic. The filter merely transforms this ammonia into nitrites, then into nitrates. The problem is that the cycle stops there for the filter: the nitrates then accumulate dangerously in the water, slowly poisoning the fish and encouraging algae growth.
What is the only genuinely biological solution? Plants. In nature, as in a filterless goldfish aquarium, only plants can consume these nitrates to grow, thereby removing them permanently from the water.
The power of floating and emergent plants (the nitrate pump)
Not all plants are equally effective at purifying the water of a heavy polluter, even though they all have a role to play. Fully submerged plants, which grow underwater, are absolutely necessary: they consume carbon dioxide, provide your fish with vital oxygen, and offer it the complex environment essential to its mental well-being (it needs a maze to explore!). However, when it comes to purification, their nitrate consumption is limited by the amount of dissolved CO2 underwater, which restricts their growth.
The secret to a self-sustaining aquarium lies in adding floating plants or plants with emergent leaves. These magical plants draw their nutrients from the water—the famous nitrates—but absorb their CO2 directly from the ambient air in your living room! No longer limited by carbon dioxide, they can photosynthesize at remarkable speed and consume nitrates at full capacity.
The absolute queen of the goldfish aquarium is Floating frogbit (Limnobium laevigatum). It is tough enough to withstand the attacks of this large, inquisitive fish. Above all, it develops long, magnificent, feathery roots that descend into the water column to absorb pollutants throughout the aquarium. Finally, it gently filters the light, making your pet feel secure without smothering the aquarium.
👉 Purify your water thoroughly with our Floating frogbit strains (Limnobium).
A living substrate based on the microbiome (not microfauna)
In the natural aquarium method (low-tech), people often emphasize the importance of microfauna—worms and small crustaceans—for maintaining the substrate. But with a goldfish, a common misconception needs correcting: it is unrealistic to hope to maintain a large visible population of microfauna in the substrate. This fish is an insatiable burrower that spends its days stirring up the sand. It will very quickly devour the water lice or ostracods you try to establish there.
However, this is not a problem! The true strength of a living substrate lies not in the small crustaceans, but in its complex, invisible microbiome. That is why I recommend using our ZollaBox Starter, even for a goldfish.
Admittedly, the microfauna it contains will serve as an exceptional welcome meal for your fish. But the essential element lies elsewhere: it is the invaluable microbiome contained in this box—billions of beneficial bacteria, microfungi, and ancient periphyton—that will become permanently established deep in your sand. These microorganisms, inaccessible to the goldfish's mouth, will recycle organic matter and balance your ecosystem over the long term.
👉 Seed the heart of your sand with the microbiome of the ZollaBox Starter.
Natural feeding and digestive well-being
If you want to keep your goldfish healthy for 15 years, you must respect its anatomy. The goldfish is an omnivorous animal with a strong herbivorous and detritivorous tendency... and it has no stomach at all!
Its digestive system is one long continuous tube. Dry industrial pellets are disastrous for it: they swell once swallowed, block its digestive transit, and cause severe inflammation of the swim bladder (the fish floats on its belly or sinks like a stone).
It needs a continuous supply of natural, moist food.
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Duckweed: this is the essential plant-based staple for its health. I recommend offering it regularly, ideally by growing a small supply at home in a separate container near a window to ensure a continuous supply of fresh greens.
👉 Give it its favorite plant-based salad bowl with our duckweed.
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Daphnia: these small freshwater crustaceans are nature's miracle remedy. Their shell is rich in chitin, a substance that acts like a fibrous brush in the goldfish's digestive tract, ensuring perfect transit and preventing blockages. To always have some on hand, it is very easy to maintain a small self-sustaining culture at home.
👉 Start your own daphnia culture with the ZollaBox Fish M.
Managing heat spikes and reference guide
Summer can be a challenging season for our animals. Here is a vital heatwave note to know: when water heats up indoors, it loses some of its ability to retain dissolved oxygen. If you see your goldfish frantically gasping for air at the surface during a heatwave in your living room, temporarily adding a mechanical air stone (air pump) can save its life by circulating the water.
Crucial clarification: if your goldfish lives in an outdoor pond, do not put an air stone at the bottom of the water in summer! The circulation would destroy the natural thermal stratification, mixing the life-saving cool water at the bottom with the scorching surface water, which would kill your fish.
To master all these subtleties of aquatic biology and offer your pet the best life possible, I invite you to consult the Official Guide to Natural Aquariums. This is not just a technical manual. It is the essential reference guide, in video format, for understanding the overall ecology of your aquarium and incorporating the concept of animal welfare—a dimension unfortunately too often overlooked in traditional aquarium literature.
👉 Understand the secrets of living ecosystems with the Official Guide to Natural Aquariums.
Conclusion: the joy of a liberated animal
Knowing which aquarium to choose for a goldfish is not just a question of liters; it is a question of ethics. Freeing this animal from its glass prison and placing it in a large, generously planted aquarium is an act of love and respect.
The joy of watching a goldfish tirelessly search through the sand, nibble on the roots of Floating frogbit, and swim with all the power of its fins in clear, filterless water is the finest reward the aquarium hobby can offer.
Mattier
FAQ: successfully transitioning your goldfish
What is the minimum water volume needed for a goldfish without a filter?
This is the golden rule for ensuring its well-being and the proper functioning of natural plant-based purification: a goldfish needs an absolute minimum of 100 liters of water per individual. These fish grow quite large—up to 30 centimeters—and produce a great deal of waste. A large volume ensures water stability and gives it the swimming space essential for muscle development. And I repeat: the absolute ideal is, of course, still a large outdoor pond.
Why does my goldfish eat my plants?
It is in its very nature! The goldfish is a grazer. If it attacks your beautiful decorative plants, it is simply because it is hungry for plant fiber and you are not providing enough of it. The solution is not to remove the plants, but to offer it tender plants intended for feeding, such as duckweed or elodea. Once it is full from these “salad” plants, it will leave your tougher plants alone, such as Philippine Anubias or hornwort.
👉 Learn more about floating plants as fish food.
How can I tell if my goldfish lacks oxygen?
A goldfish suffering from a lack of oxygen will display a very clear behavior: it will spend most of its time clinging to the water's surface, mouth wide open, loudly “gasping” for air. Its gills will beat at a highly accelerated rate. This often occurs during periods of intense summer heat or in tanks that are far too small and poorly planted. A partial water change with slightly cooler water, or the addition of an emergency air stone, will solve the problem in the short term.