If you have an aquarium or a pond, sooner or later you'll face the same question: how to keep the water well-oxygenated so that the fish, plants, and bacteria can thrive. Although it may seem simple, oxygenation in the aquarium is a fundamental pillar of biological balance, and if neglected, problems arise very quickly.
In a closed system like an aquarium, all organisms continuously consume oxygen and release carbon dioxide (CO2 ) . Fish, invertebrates, and bacteria do this 24 hours a day, while aquatic plants produce oxygen only in daylight and respire like other living things at night . Therefore, understanding the role of aquatic plants, oxygenating plants, and CO2 utilization is key to the smooth functioning of the ecosystem.
Why is oxygen so important in the aquarium?
Maintaining the amount of dissolved oxygen in the water is usually easy if the aquarium is properly set up, but when it's lacking, it's immediately noticeable: fish gasp at the surface, filter bacteria suffer, and plants stop thriving. This is because no aquarium inhabitant can live without oxygen for even a few minutes without suffering harm; to learn more, see reasons why fish die.
Fish and other animals constantly consume oxygen and release CO2 , so the balance always tends toward an excess of carbon dioxide if we do nothing. Although plants use it during the photoperiod for photosynthesis, they have a limit, and at night they also become net consumers of oxygen , just like all other organisms.
The bacteria in the biological filter, especially those involved in nitrification (such as many from the genus Nitrospira ), are major consumers of oxygen. These bacteria transform ammonia and nitrite into nitrate, processes vital for the health of the aquarium, but to do so they need a stable amount of dissolved oxygen in the water, particularly within the filter media.
If the oxygen level drops too low, nitrification slows down, toxic compounds accumulate, and the system becomes unbalanced. Therefore, ensuring a good oxygen supply at all times is considered essential for the proper functioning of any aquarium or pond.
The other side of the coin is CO2: is the “residue” of breathing de pecesinvertebrates, plants, and bacteria, and if it doesn't dissipate, it can accumulate excessively. For the ecosystem to be stable, it is necessary that expel CO2 surplus and replace the O2 lostmaintaining a reasonable balance between both gases.
What exactly are oxygenating plants?
Within the broad group of aquatic plants , there is a set known as oxygenating plants. They are not necessarily the most showy or eye-catching in an aquascape, but they fulfill a fundamental biological function in the water . In fact, in many ponds they go almost unnoticed, and yet they are the ones that maintain much of the ecosystem's balance.
The main function of an oxygenating plant is to keep the water clear and well-oxygenated. To do this, they take dissolved mineral salts and nutrients from the water, convert them into plant biomass, and release large quantities of oxygen during photosynthesis. By consuming nutrients and competing for them, they also naturally inhibit the growth of unwanted algae.
These plants also provide shelter and resting areas for fish and other inhabitants of the pond or aquarium. Their dense leaves and stems create labyrinths where fry can hide, and their roots typically anchor themselves in the mud at the bottom, where they absorb accumulated nutrients. In many species of this group, the upper part of the stem and even the flowers emerge above the water's surface , while the main leaf mass remains submerged.
In cold water aquariums, garden ponds, or small artificial pools, these types of plants become a very useful tool: they help keep the water cleaner, stabilize the parameters, and reduce the need for very complex filtration systems , provided they are given the minimum conditions of light and nutrients.
Main species of oxygenating plants for aquariums and ponds
Among the most commonly used oxygenating plants are several easy-care, high-performing species. It's helpful to understand their basic characteristics to choose the ones that best suit your setup and water conditions, as they don't all require the same care or grow at the same rate, although they all share the ability to improve water quality.
Ceratophyllum demersum is a classic choice. Commonly known as Water Pine or Foxtail, it has thin, segmented leaves that resemble underwater pine twigs. It's a very hardy plant that doesn't require substrate because it usually floats or is lightly anchored, and it adapts well to aquariums and ponds. Its rapid growth and ability to easily absorb nutrients make it perfect for beginners.
Egeria densa (also called Elodea) is another great ally. It's a very popular plant precisely because of its ease of care and its ability to combat algae, especially cyanobacteria (blue-green algae). It forms dense green clumps that, in addition to oxygenating the water, provide a very useful habitat for small fish and fry. It appreciates good light, but isn't overly demanding and thrives in both aquariums and temperate ponds.
Hottonia palustris , commonly known as water violet, is often used in aquariums as a midground plant, creating an aesthetic transition between low-growing and tall plants. Its delicate leaves and light, airy form add movement, and in ponds, it can flower out of the water. In addition to its decorative value, it constantly contributes to oxygenation as long as it receives sufficient light and good-quality water.
Among the feathery-looking plants, several species of the genus Myriophyllum stand out. For example, Myriophyllum brasiliense forms stems with highly divided leaves that lend a very distinctive texture to the aquascape. However, it is more demanding: it requires soft water, good nutrient availability, intense lighting, and, if possible, an additional supply of CO2 to maintain healthy growth.
Another interesting species is Myriophyllum aquaticum , known as the Water Palm. It grows quickly and is used in both aquariums and ponds. It's a very versatile plant: it tolerates shade or partial shade, is generally unattractive to most pests and diseases, and adapts to different water conditions. Thanks to its abundant plant mass, it efficiently contributes to nutrient uptake and oxygen release.
Orontium aquaticum is a highly decorative pond plant that, in addition to its oxygenating function, is notable for its striking inflorescences. It thrives in shallow areas and, although its maintenance requires a bit more space and stability, it combines its ornamental value with a remarkable ability to improve water quality.
Ranunculus aquatilis , commonly known as watercress, is a very common plant in Spanish waters. Its finely divided leaves are well-suited to gentle currents, and its white flowers float on the surface, creating a very attractive visual effect. It thrives in ponds and slow-moving stretches of water, where it helps to keep the water clearer and reduce algae.
Finally, Vallisneria gigantea is a favorite in large aquariums. Known as American Spider Plant, it develops long, ribbon-like leaves that can create veritable curtains on the background. It is hardy, grows quickly, and is easy to care for, provided it has some light and nutrients. Its dense foliage and vigorous growth make it a great consumer of dissolved compounds and a generator of oxygen.
How to care for and manage oxygenating plants
Most oxygenating plants are considered more functional than decorative, and generally don't require complicated care. Living almost completely submerged, they don't usually need elaborate pruning or sophisticated fertilizers, although it's advisable to monitor certain basic aspects to ensure they perform at their best.
In ponds, a density of approximately one oxygenating plant per 0,3 m² of water surface is generally recommended. This figure is a guideline, but it gives an idea of the amount of plant mass needed to keep the water reasonably clean, oxygenated, and with fewer algae . If too few plants are planted, their effect will be diluted; if too many are planted, frequent pruning may be necessary.
These plants tend to grow and multiply quite rapidly when conditions are favorable, so regular pruning is part of normal maintenance. By trimming the longest stems and removing excess plant matter, you prevent them from taking over the entire surface and taking up space needed for other plants or swimming areas for fish. In addition, removing old or damaged parts reduces organic decomposition in the water.
Lighting is another key factor. Although many oxygenating plants aren't extremely demanding, if the light is too low, their ability to photosynthesize and release oxygen is significantly reduced. It's important to provide a photoperiod appropriate for the species, adjusting the intensity and duration so that they don't receive insufficient light or experience excessive algae growth. Properly adjusted lighting parameters help the plants remain vigorous and prevent decline.
The use of nutrient-rich substrates , liquid fertilizers, conditioners, and, in planted aquariums, pressurized CO2 systems can be very beneficial. Not all oxygenating plants have the same needs, but in general, they appreciate a minimum availability of essential nutrients (nitrate, phosphate, potassium, micronutrients, etc.). Consulting the specific data sheet for each species is a good idea to fine-tune watering, lighting, fertilization, and water parameters.
Many specialized stores carry both these plants and the equipment needed to care for them: LED and fluorescent lights specifically designed for aquariums, CO2 systems , substrates, fertilizers, and all kinds of accessories. Having the right equipment makes maintaining a stable and healthy habitat much easier in the medium and long term.
Oxygenating plants and balance in ponds and aquariums
When several species of oxygenating plants are combined, a synergistic effect is achieved: growth forms, depths, and rates are diversified, and the system becomes more stable. These plants not only oxygenate the water; they also absorb nitrates, phosphates, and other nutrients that might otherwise be used by filamentous or suspended algae.
In garden ponds, fountains, and small outdoor ecosystems, aerators form part of the system's "natural filter." Along with marsh, floating , and riparian plants, they help keep the water clearer without relying solely on mechanical filters and UV lamps. At the same time, they provide numerous hiding places and reduce stress for fish and invertebrates , offering them more refuge from predators or sudden changes in light.
Plants like Ceratophyllum demersum or Elodea canadensis, widely used in ponds and cold-water aquariums, are a prime example of this utility. They are easy to install, their effect on water quality is quickly noticeable, and, if managed with pruning, they offer a very interesting balance between functionality and simplicity . Furthermore, they are especially practical in natural filtration systems or with large fish that stir up a lot of sediment.
When shipped online, they usually arrive ready to plant or simply to float. The important thing is to gradually acclimate them to the temperature and parameters of the destination aquarium or pond, avoiding thermal shock and minimizing the adaptation period . Once established, most show stable growth without major problems.
These plants also adapt to various configurations: large or small ponds, with fish, with turtles, with natural or mixed filtration systems. The key is to prevent them from being completely shaded by other plants, control their spread, and combine species to cover different layers of the water column.
Summer, aquarium temperature and oxygenation
In summer, many aquariums experience a significant temperature increase. The problem isn't just the heat itself, but also that warm water can dissolve less oxygen. In other words, just when fish and bacteria consume more oxygen due to increased metabolism, the water is able to retain less dissolved oxygen , disrupting the balance.
In typical tropical aquariums, ideal temperatures usually range between 24 and 27°C, while more delicate species like discus or certain shrimp thrive in slightly higher but very controlled ranges. Most aquatic aquarium plants are comfortable around 23-26°C. When the temperature rises above these values, measures must be taken to avoid prolonged heat spikes.
To lower the temperature a few degrees, you can use fans to circulate air across the water's surface, promoting evaporation and dissipating heat. In some cases, frozen water bottles have been used, although this is an emergency measure and must be applied carefully to avoid sudden temperature changes . A temperature controller that automatically turns fans and heaters on and off is very helpful in maintaining the aquarium within safe temperature ranges.
Lighting should also be adjusted in summer. Too many hours of light, combined with the natural light coming in through the window, often triggers algae growth and raises the water temperature. Reducing the photoperiod to about 6-8 hours per day, using timers, and preventing the aquarium from receiving direct sunlight are simple steps that reduce stress for both plants and fish.
Evaporation accelerates with heat, so it's common for the water level to drop rapidly. When refilling, use soft or reverse osmosis water to avoid increasing the salt concentration and sudden changes in hardness (GH and KH). More frequent level checks, along with more regular cleaning of the glass, filter, and substrate, help prevent the system from becoming overloaded with decomposing organic matter.
Practical oxygenation: surface, bubbling and plants
The most direct way to improve oxygenation in an aquarium is to increase gas exchange with the atmosphere. This is achieved by moving the water surface through the filter outlet or with the help of air pumps and bubble diffusers. When the water surface is almost completely still, gas exchange is reduced, and the dissolved oxygen level can drop more rapidly overnight.
Directing the filter outlet slightly upwards, so that it agitates the surface without creating excessive waves, is usually a good solution. This agitation promotes oxygen intake and the release of CO2 into the atmosphere. In aquariums with a high CO2 injection , the exact point needs to be fine-tuned, as too much turbulence can dissipate the carbon dioxide that plants need too quickly.
Air compressors with bubble diffusers are a classic tool for boosting oxygenation. The bubbles not only provide some oxygen, but as they rise, they create movement in the water column and improve the homogeneity of the gas mixture . In summer, or in densely populated aquariums, they can make the difference between a stable system and one on the brink.
In addition to mechanical equipment, oxygenating plants themselves are a huge help. Adding species like Elodea or Limnophila to a community aquarium enhances the system's ability to consume nitrogenous waste and produce oxygen during the photoperiod . However, they don't completely replace good filtration or proper water movement, but rather act as a natural complement.
Overpopulation should be monitored, especially during warm weather. The more fish there are, the greater the oxygen consumption and CO2 production.2 and waste. Slightly reduce the amount de peces When the water is warmer, it also helps prevent the buildup of organic matter, which in turn oxygen demand during decomposition.
CO2 facing O2Differences and relationships in planted aquariums
In planted aquariums, in addition to oxygen, considerable attention is usually paid to CO2 , as it is one of the main nutrients for photosynthesis. The injected carbon dioxide allows plants to grow faster, develop more vibrant colors, and create much denser and more striking underwater landscapes . However, managing it requires a thorough understanding of its relationship with oxygen.
CO2 is supplied using specific systems: pressurized cylinders, pressure regulators, bubble counters, check valves, and diffusers that help dissolve the gas in the water. To monitor the concentration, pH and KH tests or devices like the drop checker, which changes color depending on the amount of dissolved CO2, are used . Maintaining a stable and appropriate concentration is crucial, because an excess can be lethal to fish.
Oxygen , on the other hand, is introduced more indirectly: through surface movement and the action of air pumps and diffusers, in addition to the plants' own photosynthesis. Unlike CO2 , O2 does not usually require such fine-tuned regulation systems, although it should not be overlooked, especially in aquariums with a high biological load or heavily planted tanks, where the oxygen demands of bacteria and fauna are significant.
There is an inverse relationship between these two gases: when the concentration of CO2 increases , the amount of dissolved oxygen tends to decrease, and vice versa. This doesn't mean they can't coexist at suitable levels, but rather that a reasonable equilibrium must be found where plants have enough CO2 to grow, while fish have enough oxygen to breathe without problems.
A common practice in aquariums with pressurized CO2 is to turn on the system approximately half an hour before the lights are switched on. This ensures that when the plants begin their photosynthesis, there is already a sufficient concentration of carbon dioxide in the water. Similarly, it's usually a good idea to turn off the CO2 shortly before switching off the lights , so that the gas doesn't accumulate unnecessarily when the plants are dormant.
It is also very important to avoid sudden changes in the amount of CO2 injected . Large adjustments from one day to the next can cause spikes that stress fish and plants. Ideally, the dose should be increased or decreased gradually, observing the aquarium's reaction and always maintaining a certain stability in both CO2 and oxygen levels.
Practical tips for balancing CO₂2 and oxygenation
In a densely planted aquarium with CO2 injection2It is generally recommended to keep the water surface relatively still during the photoperiod to minimize the loss of carbon dioxide to the atmosphere. However, this must be done carefully, as A pond of almost still water can suffer from poor oxygenation. if the population de peces is elevated.
A commonly used strategy is to slightly increase surface agitation at night. This compensates for the fact that plants, not photosynthesizing in the dark, consume oxygen like all other living organisms. This approach prioritizes CO2 when plants need it most (in the light), and favors oxygenation when O2 demand exceeds production.
Surface skimmers can also be helpful in planted aquariums. These devices remove the oily film, or biofilm, that sometimes forms on the water's surface and can reduce gas exchange. By removing it, oxygenation is improved, and the surface layer is prevented from reducing transparency and hindering light transmission into the aquarium.
If, despite everything, signs of oxygen deficiency are observed (fish breathing rapidly, clustered near the surface, lethargy), the wisest course of action is to increase water movement or add an aerator, even if this means losing some CO2 . The health of the fish and the nitrifying bacteria should always be the top priority, as without them the entire system will collapse.
Often, the details make all the difference: a regular maintenance routine along with the correct filter outlet height, good distribution of oxygenating plants, and sensible population control de peces These are sufficient for the aquarium to maintain adequate oxygen and CO2 levels.2 very manageable, without the need for major technical complications.
When oxygenating aquatic plants, adequate surface oxygenation, good CO2 control, and seasonal maintenance (especially in summer) are intelligently combined , the aquarium becomes a much more stable, visually appealing, and easy-to-enjoy ecosystem on a daily basis, with active fish, vigorous plants, and clear water that reflects that the whole system is truly in balance.
