Why are my fish swimming or gasping at the water surface?

Why are my fish swimming or gasping at the top of my tank?

Many fish keepers in our hobby wake to find their beloved aquarium or pond inhabitants swimming or gasping at the surface of the water, and will assume that its because they cannot get enough oxygen. However, fish gasping at the top of the tank or pond is a common problem that can be caused by several factors.


Yes, the most common reason is low oxygen levels in the water. When the oxygen levels in the water are low, fish will swim to the surface to get more air. Often if the temperature is too high, or the current circulating too low, there will be less oxygen present in the water. Creating some surface movement, or installing an aquarium air pump or pond aerator with an air stone attached will often resolve these problems.

air bubbles in a goldfish tank

Other reasons for fish gasping at the top of the tank include high ammonia or nitrite levels, high water temperature, overcrowding, and diseases.

Water Quality

Water parameters can change rapidly and have many contributing factors. An increase in waste from new inhabitants, or a drop in filter capability or a fault can all be causing levels to change over time, and harmful toxins to build up.

The most common water quality issue is a raised level of ammonia, nitrate or nitrite. Use a test kit to see if any of these are at harmful levels, and take steps to address the issue. Improving aeration, circulation, filtration, doing a partial water change or adding good bacteria and water clarity treatments are all ways to address water quality.

Sometimes the pH level can change if you have moved home, used rain water or installed a water softener, which can stress and irritate some fish. Check your fishes pH requirements before you buy - their tolerance will often be listed on online fish listing descriptions or on information labels in stores. A change in pH may not be the cause of fish gasping, but it could attribute to other causes, like disease, if a change has caused stress to the fish.

fish in dirty aquarium water


High Temperature

When an aquarium is at a high temperature, the water becomes less dense and will hold less oxygen. Sometimes the temperature will increase in the aquarium to levels that your fish cannot tolerate.

The main contributing factors to this are either a malfunctioning heater or a thermostat set too high, an increase in room temperature on hot days, or direct sunlight hitting the glass. Remember, a fish tank will rarely be cooler than the room it is situated in.

Good circulation and air bubbles will help disperse the heat and increase oxygen levels. Inspecting and turning down heaters, lowering the ambient room temperature and blocking direct sunlight are all factors in lowering room temperature. In extreme cases, adding ice or cold water to your pond or aquarium will help lower the temperature.


A fairly obvious one this - if an aquarium is overcrowded, the fish will be releasing more waste which can lead to a harmful build-up of ammonia and nitrate/nitrite. If a filter struggles to process the waste, pollutants will increase and you may experience gasping issues.

Also, the more fish you have, the more oxygen they are likely to consume. Adding an air pump to your aquarium setup or adding live aquarium plants will increase oxygen levels and decrease pollutants.

fish gasping at fish tank surface

If you suspect that your pond or aquarium is overcrowded, upgrade your filtration or take practical and responsible steps to reduce the numbers. For a guide to the amount of fish you should stock in your fish tank, have a look at our Fishkeeping Guide for formulas and helpful advice.


Fishes that are stressed or for whatever reason have low immunity are highly susceptible to disease from pathogens present in the water. Common diseases like Ich, or white spot, are prevalent in all water generally and will quickly establish when given the opportunity.

Disease can often be a sign of a poorly kept aquarium with poor water quality or something not quite right in the tank. It may be that fish are incompatible and attacking each other, or it could simply be that the lights are left on too long. Sometimes it can be a disease introduced by new fish that have not been quarantined properly prior to introducing.

Identifying and treating fish diseases is something that should be done properly and never guessed at. Take the time to explore the internet for similar photographs of what you have, or ask a local vet or aquatic expert for advice. Never start a treatment program without properly identifying the disease, as this can do more harm than good.

Addressing the overall cause and administering treatment is the solution to disease. It may not be the cause of fish gasping at the water surface, but it may well be a symptom or result of the water quality or habitat issues contributing to it.

 fish with ich white spot disease

If you suspect a fish has a disease, quarantine the fish and administer a treatment to both the sick fish in quarantine and the remaining livestock in the tank. All disease treatments come with instructions for treating common disease.


Its possible, although unlikely, that fish will be gasping at the water surface because they are hungry or have noticed you are about to feed them. Fish are very intelligent, and will often rise at feeding time or when you approach. This is harmless and normal behavior, so if you find that the fish settle after being fed and do not remain at the surface, problem solved.


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Photo credit(s): Canva Pro Licence, Microsoft Bing AI Image Creator.

AquariumsDiseaseFish healthFish tanksKnowledgePondsWater quality

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