Fish gasping at the surface: what to check
Updated on August 26, 2026 · ~6 min read
A fish gasping at the surface cannot take up oxygen: either because there is little in the water (too warm, poor flow, high CO₂) or because its gills are not working (ammonia, nitrite, parasites). The first two measures are increasing surface movement and measuring ammonia and nitrite.
It is the most alarming symptom and one of the most informative. A fish hanging motionless just below the water surface, its mouth opening and closing rapidly, is looking for oxygen at the one point in the tank where it is slightly more concentrated: the interface with the air.
The causes fall into two families, and the distinction decides what to do:
- There is little oxygen in the water. Heat, poor flow, high CO₂, overstocking, a bacterial bloom.
- The oxygen is there, but the fish cannot use it. Nitrite in the blood, gills damaged by ammonia, gill parasites.
The second family is the one where an air pump does not help, and it is also the most frequent in a new tank.
The first five minutes
Before working out which of the seven causes it is, two moves that do no harm in any of them:
1. Increase surface movement. Aim a pump upwards so it breaks the surface, or switch on an air pump. Gas exchange happens at the surface: the more surface in motion, the more oxygen goes in and the more CO₂ comes out. In half of all cases the picture improves within half an hour.
2. Shut the CO₂ off, if there is injection. Runaway CO₂ is one of the fastest and most lethal causes, and shutting it off costs the plants nothing for a day.
Then you measure, instead of guessing.
The seven causes, in order of likelihood
1. Nitrite (new tank or damaged filter)
Nitrite binds to haemoglobin and stops it carrying oxygen: the fish suffocates in saturated water. It is the most common cause in tanks less than two months old, and the one in which an air pump helps least.
The correct value is 0 ppm. Above it: repeated 30-50% water changes, food suspended, no new arrivals. See high nitrite.
2. Ammonia
It burns the epithelium of the gills, which stop exchanging gas. Here too the correct value is 0 ppm, and here too the answer is a water change. ⚠️ With ammonia in the tank do not raise pH: the toxic NH₃ fraction increases with pH and temperature, so "sorting out the parameters" in an emergency can make everything worse in one move.
3. Temperature too high
Warm water holds less oxygen, while the fish's metabolism — that is, their consumption — rises: the two curves go in opposite directions. It is the dominant cause in summer. The reference is 24-27 °C (75-81 °F) in a tropical freshwater tank; above 29-30 °C (84-86 °F) you are in the critical zone.
You bring it down gradually: a fan across the surface (2-3 °C / 4-5 °F, and that is nearly always what is needed), lights reduced, lid open. Never cold water poured in.
4. Excess CO₂
In a planted tank with injection, a dose that got away or a diffuser no longer offset by surface exchange. The tell: it happens during the lit hours, and the evening is better. The reference is 20-30 ppm with injection; and the rule of prudence is a single one — if the fish stay at the surface with their mouths open, shut the CO₂ off and increase movement: the animals' safety comes before the plants.
5. Overstocking or poor flow
Too many fish for the volume, or a long narrow tank with a weak pump in one corner. Here the symptom is chronic, not acute: the fish sit high most days, not since yesterday. The check is the tank's load and the position of the pumps.
6. Bacterial bloom
Water that has suddenly turned milky white means bacteria in suspension, which consume oxygen. It resolves by itself over days; in the meantime the only thing to do is oxygenate and cut back the food. See cloudy water.
7. Gill parasites or infection
If the values are fine, the temperature is right, there is flow and only one fish is gasping while the others are well, suspicion moves to the animal: gill parasites, a bacterial infection, an injury. Signs that help: gills held open or reddened, asymmetric breathing (only one operculum moving), the fish rubbing itself.
How to tell them apart: three questions
Are they all gasping or just one? All → the water. One only → the animal.
Is it worse during the lit hours or in the morning? Worse with the lights on → CO₂. Worse in the early morning → low oxygen overnight (the plants consume it), often together with overstocking or poor flow.
Did it start yesterday or has it been going on for weeks? Sudden → an event: CO₂, a stopped filter, temperature, a dead animal. Chronic → a structural limit: load, flow, volume.
What not to do
- Do not feed. A fish in respiratory distress should not be digesting, and uneaten food adds to the organic load.
- Do not add medications at random. Many treatments consume oxygen or damage filter bacteria: on a fish that already cannot breathe it is the worst move. Numbers first.
- Do not change the water with cold water "to cool things down". The swing is a second stress on an animal that already has one.
- Do not treat the air pump as a diagnosis. If it improves things, the cause was oxygen; if it does not, that does not mean you need more of it — it means the problem is in the blood or the gills.
The value worth knowing, if you have the test. Dissolved oxygen sits around 6-8 ppm in freshwater and 6-7 ppm in marine (salt lowers saturation). Below about 4 ppm you are in an emergency: increase surface movement, lower the temperature, suspend CO₂ and any treatments that consume oxygen. It is not a test everyone owns, and that is fine: the first six causes are diagnosed without it.
How Gurglee does it
Faced with this symptom you need three numbers quickly — ammonia, nitrite, temperature — and Gurglee lets you log them by writing them the way you would say them ("ammonia zero point two five, nitrite half"), telling you straight away which are outside that tank's band. The parameter's page also shows the predicted effect of a water change before you make it, so you know whether a 30% change is enough or whether it takes two. And you can ask the tank doctor, which answers by reading that tank's actual readings instead of giving generic advice.
Related guides
High nitrite in an aquarium: what to do right now
Nitrite above zero: why it is an emergency, what to do in the next few hours and the next few days, and how long it takes to fall back to zero on its own.
Ammonia in an aquarium: why it is the most dangerous
What ammonia is in an aquarium, why it kills, where it comes from and what to do when the test detects it. With the role of pH and temperature in its toxicity.
Aquarium temperature: which one for which fish
What temperature to hold for each type of tank and fish, why swings matter more than the value, and what to do when the water gets too warm in summer.
New tank syndrome: symptoms and what to do
Fish in a new tank are sick or dying: how to recognise new tank syndrome, what to test straight away and how to limit the damage.