Why Are My Goldfish At The Top Of The Tank? (Explained!)

There are 3 main reasons a goldfish may be gasping at the surface. These are low oxygen levels in the water, high ammonia, nitrite, or nitrate levels in the water or the goldfish has a pest or disease which is affecting its ability to breathe normally.

Goldfish have been kept as pets for hundreds of years. Since at least 1650, people have been keeping and selectively breeding goldfish. From a tiny bowl to giant ponds, almost every fishkeeper has had a go at keeping goldfish at one point or another.

Eating Daphnia
Eating Daphnia

I personally love fancy goldfish. I keep a large tank of fantails. They bumble around the tank all day long, just looking for the next thing they can eat. The problem with goldfish is they get much larger than people think, we typically keep them in aquariums that are too small. Both of these problems can lead to a goldfish gasping at the top of the tank.

Why Do Goldfish Gasp At The Surface?

Even though goldfish live in water, they still breathe oxygen just like we do. The only difference is goldfish absorb the oxygen directly out of the water using their gills. When oxygen levels in the water drop, the goldfish will head for the surface.

Although it may look like they are gasping and trying to breathe the air, they are in fact at the surface because that is where the highest concentration of dissolved oxygen in the water can be found.

A few years ago I went to a buddy’s house and he had a tank full of goldfish that were all gasping at the surface. He didn’t realize it was because they were struggling to breathe, he genuinely thought the goldfish were begging for food!



What Affects Available Oxygen In The Water?

All water has a natural amount of oxygen and carbon dioxide dissolved in it. Fish absorb the oxygen through their gills and release carbon dioxide just like we do. At the water surface, there is a natural process that takes place whereby gases are exchanged between the water and the air above.

When the water surface is still and calm, there isn’t much exchange of gases. When the water is moving rapidly there is a higher exchange of gasses. Essentially, water that has a rapidly moving surface will generally have a higher level of dissolved oxygen in it.

Bearing the above information in mind, we know that if our aquariums have a lot of surface movement, there is a good chance there will be sufficient dissolved oxygen. Conversely, if there is little to no surface movement, the likelihood is there will be less dissolved oxygen.

There is another way to increase the amount of dissolved oxygen in the aquarium, and that is through live plants.

Live aquarium plants, just like terrestrial plants, absorb carbon dioxide during the day and expel oxygen. Keeping live plants is something I highly recommend, even in goldfish tanks. I currently keep 3 large fantails in an aquarium with cryptocoryne wendtii and a very large clump of Java Moss.

It should be noted that plants alone may not be enough to keep dissolved oxygen levels high enough as, when the lights go out, plants start to absorb oxygen and release carbon dioxide instead.

How Do I Increase Oxygen In My Fish Tank?

There are essentially two recognized ways to increase the amount of dissolved oxygen in your goldfish aquarium water. The first is by adjusting your filter output and the second is by adding an air pump.

Adjusting Filter Output

Whether you are filtering your aquarium with a canister filter, a hang-on-back filter or an internal power filter, at some point the water has to return from the filter back to the tank. It is at this point that maximum effort should be made to agitate the water surface.

With canister filters, there are usually two choices, a return nozzle or a spray bar. In my experience either will work just as well. If you are using a spray bar, position it just above the water surface so you get as much agitation of the water surface as possible.

As the water sprays back into the aquarium it will drag some air with it and move the water surface, increasing the transfer of gases. With a return nozzle, position it right at the water’s surface so you achieve maximum agitation and surface movement without excessive splashing.

With hang-on-back filters, there are generally no options when it comes to the water returning, it just falls from the filter back into the aquarium. This action causes lots of surface movement and allows for gas exchange to take place.

Internal Power filters return the water much like a canister filter nozzle. Position the filter right at the water’s surface for maximum surface movement.

Adding an Air Pump

No matter what filter you are using, adding an air pump with an airstone attached will cause additional surface movement which will seriously increase the amount of dissolved oxygen in your goldfish tank.

Many people believe the bubbles rising dissolve into the water, but in reality, it is the ‘popping’ at the surface which increases the oxygen levels. I seriously believe EVERY aquarium should have an air pump and airstone running, no matter what the setup.

I have had great success using these air stones that I ordered from Amazon.



How Can I Reduce Ammonia, Nitrite And Nitrate Levels In The Water?

Goldfish excrete their waste directly into the aquarium water. This waste is very high in ammonia, which is toxic to fish. Left untreated, this ammonia will quickly kill a goldfish. To deal with the fish waste we use an aquarium filter. The filter’s job is to convert the ammonia firstly to nitrite, which is less toxic than ammonia, then to nitrate which is less toxic still.

No matter how good your aquarium filter is, it will only convert the fish waste to nitrate. The nitrate will build up over time to the point where it becomes toxic. It is our job as fish keepers to reduce that nitrate by carrying out regular water changes.

If your goldfish are gasping at the surface, but you have plenty of surface movement, check your ammonia, nitrite, and nitrate levels using an aquarium water test kit. I like to use the Master Test Kit which is made by API (I found this one on Amazon to be the best value). I have found it to be easy to use, reliable, and fairly accurate.

If after testing your aquarium water you find your ammonia, nitrite, or nitrate levels are elevated, carry out a large partial water change ASAP. I would recommend changing about 50% of the aquarium water. This water change will reduce the ammonia, nitrite, or nitrate.

After a day, retest the water. If the ammonia, nitrite, or nitrates levels are still too high, repeat the 50% water change.

In Conclusion

Goldfish are usually at the very top of the tank if they are struggling for oxygen. Most of us can’t test for the amount of dissolved oxygen in our aquariums, but we can test for other possible issues like high ammonia, nitrite, or nitrate.

Almost no matter what the cause of goldfish at the surface, carrying out large particle water changes will help solve the problem, at least in the short term.


About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James
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