Fish gasping at the surface: causes and what to do now
Fish gulping at the top of the tank are short of oxygen or have damaged gills. The ten-minute routine, and how to tell low oxygen from ammonia or heat.
By Xavier Jodoin
In short
Fish gasping at the surface are short of oxygen, either because the water holds too little or because their gills cannot take it up. Break the surface first: point the filter outlet upward or add an air stone. Then read the thermometer and test ammonia and nitrite, and change half the water if either reads above zero. If every fish is up there, the cause is the water; if it is one fish, look at its gills.
What gasping looks like
The fish hang just under the surface with their mouths at the film, gulping. Gill covers pump fast and wide. They stay there, ignoring food and each other, and drift back up within seconds if something startles them down. Fishkeepers call it piping, and it means one thing: the fish are not getting enough oxygen, and the thin top layer of the tank is where most of it is.
There are only two ways that happens. Either the water does not hold enough oxygen, or the gills cannot take up what is there. The first is a tank problem and usually takes minutes to fix. The second is ammonia, nitrite, chlorine or disease, and the test kit tells you which. You do the same first step for both, so start before you know.
What is not gasping
Bettas and gouramis breathe air on purpose. They have a labyrinth organ, and a trip to the surface every few minutes for a gulp is normal for them. A betta that hangs at the top pumping its gills between gulps is a different matter.
Corydoras and some loaches dart up for a mouthful of air now and then and go straight back down. Once in a while is normal; every few seconds is not.
Feeding time. Fish that rush to the top when you walk up, mouths working, and scatter the moment the food is gone are begging.
The difference is that gasping fish stay at the surface, breathe hard and do not care about food.
- 1
Every fish at the surface, readings clean
Low oxygen. Break the surface: raise the filter outlet or add an air stone
- 2
Worst when the lights come on, better by midday
Plants and algae used the oxygen overnight. Run an air stone in the dark
- 3
Thermometer above 30 °C
Warm water holds less oxygen. Cool it slowly, a degree or two an hour
- 4
Ammonia or nitrite above zero, gills red or brown
Gill damage. Change half the water now and test again in a few hours
- 5
One fish only, water tests clean
Gill disease or parasites. That fish needs a checkup, not more air
The first ten minutes
Do these in order. The first two cost nothing and cannot make anything worse.
1. Break the surface. Oxygen gets into a tank through the surface, and only when the surface moves. Turn the filter outlet so it ripples the top, or lower the water level a few centimetres so the return splashes. Add an air stone if you have one. With no pump at all, scoop tank water in a jug and pour it back from a height, and keep going. Most tanks short of oxygen improve within ten minutes of this.
2. Read the thermometer. Warm water holds less oxygen and makes fish need more of it. Above 30 °C in a tropical tank, the heat is at least part of the problem: unplug the heater, switch off the lights, open the lid and stand a fan across the surface. Bring it down by a degree or two an hour, not faster. A heater stuck in the on position is a common cause, and so is a heatwave.
3. Test ammonia and nitrite. Both should read zero. If either does not, change half the water now, with a dechlorinator and matched temperature, and test again in a few hours. The ammonia spike guide has the routine for the next day and the next week.
4. Think back an hour. If the gasping started right after a water change, the new water was not dechlorinated, or it was much warmer or colder than the tank. Dose dechlorinator for the full tank volume straight away; it works in minutes. If you sprayed air freshener, polish or insecticide in the room, or had soap or lotion on your hands, change half the water and run fresh carbon in the filter.
5. Stop feeding for a day. Digestion uses oxygen, and uneaten food uses more as it rots.
In the app — log the temperature, ammonia and nitrite and the verdict says which one is the problem and how far off it is, with the water change already sized to your tank. The trend chart shows whether this has been building for days or arrived this morning.
Low oxygen: every fish, and worse in the morning
When the whole tank is at the surface and the readings are clean, the water is short of oxygen. The usual reasons:
The filter stopped or slowed. A power cut, a clogged sponge or an intake blocked by a leaf. The flow that rippled the surface is gone, and so is the oxygen it carried to the bacteria in the filter.
The surface is still. An outlet set well below the waterline, a tight lid, or an oily film from food or hands. A still surface exchanges very little gas however clean the water is.
The tank is warm. Water at 30 °C holds about a sixth less oxygen than water at 20 °C, and the fish in it breathe faster.
Too many fish, or too much life. A heavily stocked tank uses oxygen faster than a quiet surface replaces it. So does a bacterial bloom, the milky haze in a new tank, or a thick algae bloom.
The lights were off. Plants and algae make oxygen in the light and use it in the dark. A densely planted tank, or one with green water, can be fine at midday and short of oxygen by dawn. Fish that gasp when the lights come on and recover by lunchtime are telling you this. Run an air stone overnight.
A treatment is in the water. Some medications and most algae treatments lower dissolved oxygen, and their labels say to add aeration for that reason. In a planted tank with injected CO2, a regulator that drifted has the same effect: turn the gas off and aerate.
Gills that cannot use the oxygen
When the surface is moving and the fish still gasp, the gills are the problem.
Ammonia burns them. They look red and inflamed, and the fish may dart or rub against things as well. Any reading above zero in a tank with fish is the cause until proven otherwise.
Nitrite gets into the blood and stops it carrying oxygen, so a fish gasps in water that has plenty. The gills turn brown instead of red. Change half the water, and add plain aquarium salt, about a teaspoon per 20 litres, which blocks nitrite at the gills. Leave the salt out with scaleless fish and live plants. The parameters guide explains both readings.
Chlorine and chloramine from untreated tap water burn gills within minutes. Dechlorinator fixes the water at once; the gills take days to heal.
Disease, when it is one fish or a few and the water tests clean. Gill flukes, ich and velvet all settle on the gills before you see anything on the body, and columnaris attacks them directly. Look for one gill cover held open or clamped shut, rubbing against rocks, clamped fins, or a fine dusting on the skin under a torch. That fish needs a diagnosis, not more air.
In the app — a checkup with a photo, your readings and Gasping at surface ticked gives Finley the whole picture. He compares the symptom with your water and says whether this looks like oxygen, a toxin or the gills, and what to do first.
Stopping it happening again
Keep the surface moving all the time, not only in an emergency. A filter outlet at the waterline or a small air pump is enough.
Rinse the filter sponge in a bucket of tank water once a month, before the flow drops, and check the intake when you feed.
Stock for the surface area you have. A tall, narrow tank holds fewer fish than a long, shallow one of the same volume, and the stocking guide works through the numbers.
In summer, watch the thermometer rather than the heater setting. On hot days open the lid, cut the lighting hours and add an air stone before the fish ask for one.
Dechlorinate every drop of tap water, every time, and keep sprays out of the room.
Common questions
Why are my fish gasping at the surface after a water change?
Usually chlorine or chloramine in tap water that was not treated, or new water several degrees warmer or colder than the tank. Add dechlorinator for the whole tank volume straight away and make sure the surface is moving. If you did treat the water, test ammonia: a filter rinsed under the tap during the same clean loses its bacteria, and the spike arrives a few hours later.
Why are my fish at the surface in the morning but fine later?
Plants and algae use oxygen in the dark and make it in the light, so a planted tank or one with green water is at its lowest just before the lights come on. The fish recover as the day goes on. Run an air stone overnight, or raise the filter outlet so the surface ripples around the clock.
Is it normal for a betta to go to the surface for air?
Yes. Bettas and gouramis have a labyrinth organ and take a gulp of air every few minutes by design, and a healthy betta does it calmly and swims off. It is a problem when the fish stays at the top with its gills pumping hard, which points to cold water, ammonia or gill disease in the same way it would for any other fish.
Do I need an air pump in my aquarium?
Not if the filter outlet ripples the whole surface, because that movement is what lets oxygen in. An air pump is worth having in warm weather, in a heavily stocked or heavily planted tank, during most medications, and as a spare for the day the filter stops. The bubbles do very little themselves; the surface they disturb does the work.
Can fish recover after gasping at the surface?
Usually, if the cause is fixed within hours. Fish short of oxygen from a still surface or warm water are often back to normal the same day. Gills burned by ammonia, nitrite or chlorine take one to two weeks to heal, and the fish is easy prey for infection in that time, so keep the water spotless and feed lightly.