Why do fish float on the surface of a pond?

Fish swims on the surface of a reservoir when the concentration of dissolved oxygen in the water drops below a critical level of 3-4 mg/l. This is a physiological necessity for the survival of aquatic organisms trying to capture atmospheric air or find more oxygenated layers. Water conditions are changing rapidly, and ignoring the first signs hypoxia can lead to mass mortality of the entire livestock in a matter of hours.

The behavior of fish rising to the upper layers of the water often signals systemic problems with filtration or overcrowding of the aquarium. In natural bodies of water, such phenomena are observed before a thunderstorm or during the period of water bloom, when algae absorb all available oxygen at night. For aquarists, this is a direct signal to take immediate action to aerate and check environmental parameters.

Oxygen starvation as the main reason

The main reason why fish floats on the surface of a reservoir is a simple lack of oxygen dissolved in the water. This process is called hypoxia, and it develops faster in warm water because the liquid's ability to retain gases decreases as the temperature rises. If you notice that the inhabitants of your pond or aquarium swallow air mouth, this is a critical symptom.

In the summer heat, photosynthesis of algae supersaturates the water with oxygen during the day, but at night the plants begin to actively absorb it, causing sharp fluctuations. The lack of mechanical aeration or powerful filtration in a closed system makes the situation worse. Fish are forced to rise to the surface, where natural gas exchange with the atmosphere occurs.

• 🐟 High water temperature reduces oxygen density.
• 🌡️ Overpopulation of the reservoir increases oxygen consumption.
• 🌿 Blooming water blocks light and disrupts photosynthesis.

It is urgent to check the operation of the compressors and, if necessary, increase their power. In ponds, it is worth using fountain aerators or cascades for forced mixing of water layers.

Poisoning with ammonia and nitrogenous compounds

The toxic effects of ammonia often cause fish to float to the surface in an attempt to escape the toxic environment. Ammonia is released as a waste product of fish and during the decomposition of organic matter, becoming deadly even in small concentrations. When fish are poisoned, their gills are damaged, making it difficult to breathe normally even in oxygenated water.

Visually, poisoning can be determined by redness of the gill covers and sluggish behavior when the fish floats on the surface of the reservoir almost motionless. This is often accompanied by cloudiness of the water or the appearance of an unpleasant smell of rot. In such cases, biological filtration can no longer cope with the load.

⚠️ Attention: High levels of ammonia cause chemical burns to the gills and mucous membranes, which requires an immediate change of 30-50% of the water volume.

To reduce toxicity, it is necessary to strengthen filtration and use special conditioners that neutralize ammonia. Regular siphoning of the soil helps remove accumulated organic matter, which is the source of the problem.

Infectious diseases and parasites

Parasitic infestations such as gyrodactylosis or ichthyophthyriasis can interfere with the swim bladder and respiratory system. When a fish floats sideways or belly up on the surface of a body of water, it often indicates a loss of buoyancy control. Parasites damage the gill apparatus, causing oxygen starvation even with normal gas concentrations in the water.

Bacterial infections can cause dropsy, which causes the fish to swell and become unable to submerge. In this case, the swim bladder is compressed by internal pressure, pushing the fish upward. Treatment requires isolation of sick individuals and the use of broad-spectrum antibiotics.

• 🦠 Damage to gill filaments by parasites.
• 💧 Inflammation of the swim bladder due to infections.
• 🚫 Impaired coordination of movements and orientation in space.

A careful inspection of the fish will reveal white spots, plaque or sores on the body. Early diagnosis allows saving a large part of the population before the outbreak of an epidemic.

Swim bladder injuries and mechanical damage

Mechanical injuries resulting from fights or hitting decorations often lead to a violation of the seal of the swim bladder. If the fish floats on the surface of the pond and cannot sink to the bottom, the bladder is probably full of gas or damaged. This condition is not always fatal, but makes the fish vulnerable to predators and stress.

Overeating dry food also causes the food to swell in the stomach, which puts pressure on the swim bladder. When dry granules fall into water, they absorb liquid and increase in volume, squeezing internal organs. Therefore, it is important to pre-soak the food before adding it to the pond.

In some cases, temporary fasting and lowering the water level can help to reduce pressure on the fish's body. However, with serious tissue ruptures, the prognosis may be unfavorable.

Influence of atmospheric pressure and weather conditions

Sharp changes in atmospheric pressure affect the solubility of gases in water and the well-being of aquatic organisms. Before a thunderstorm, pressure drops and fish may rise to the upper layers, where oxygen levels are higher, or due to a change in buoyancy. This is a natural behavior that does not always require human intervention.

The wind mixes the surface layers of water, saturating them with oxygen, so in windy weather fish often stay near the surface. However, if calm persists for a long time in the heat, the risk of death (oxygen starvation) increases many times over. The critical moment is at night in calm weatherwhen photosynthesis stops.

Aquarists should be aware that pressure surges can cause stress in sensitive species. During this period, it is better not to clean or replant plants, so as not to aggravate the load on the fish’s body.