Can Fish Drown? Your Questions Answered

Fish and water go together like…Well, a fish in water! The last thing we’d ever consider is a fish drowning in the fluid it needs to survive!

But what if I told you that in certain circumstances this can happen? Fish are sometimes in very real danger of drowning – and it’s worth talking about as it impacts how we take care of fish in the aquarium hobby!


Can Fish Drown?

Let’s get one thing out of the way first: what we mean by “drowning.” When a person or other air-breathing animal drowns, water fills the spaces needed for air. Eventually the animal suffocates because gases can no longer diffuse properly.

Fish don’t have this issue because they use gills. These feathery, blood-vessel packed structures are more like a fine screen than our fleshy, foamy lungs. Water can flow relatively unimpeded yet they still provide plenty of surface area for oxygen, carbon dioxide, ammonia, and other chemicals to be exchanged with the environment.

Incidentally, it’s always a good idea to inspect your fish’s gills whenever you get a spare moment. Discolored, pale, or dark brown gills can indicate nitrate poisoning, bacterial infections, and other disorders that need your attention.

So fish don’t “drown” since water is their preferred medium. The only way to really “drown” a fish would be to take it out of the water. Despite there being many times more oxygen in the atmosphere gills lack the support and moisture they need to properly function.

Fish can therefore drown in air if you don’t get them submerged fast enough. Incidentally, how fast fish drown in air depends on their metabolism, size, and how well adapted they are to low-oxygen conditions.

Slow bottom feeders that live in still ponds like carp and catfish can survive for hours as long as they are kept moist. On the other hand, stream-dwellers like trout and tetras will suffocate in a few minutes.


How Else Can Fish Drown?

Fish, like humans, need oxygen to survive. While we get our oxygen from the air, fish breathe water and extract the same O2, just dissolved instead.

Water contains far less oxygen than air does. Precisely how much depends on the salinity, temperature, and other factors. However it can be as little as 100 times less oxygen by volume.

So can fish “drown” in water? If, by drowning, we mean they can run out of oxygen in the water and suffocate, the answer is…Yes! While it doesn’t happen very often, anything that takes oxygen away from the water column can cause fish to suffocate and die.

While there are many ways this can occur, here are some of the most common ways fish drown in aquariums.

Too Little Gas Exchange

Since there is much more oxygen in the air than in the water gas needs to dissolve in order for fish to use it. While still water can still allow gas exchange, disturbing the surface with flow, splashing, turning over saturated and unsaturated zones, and adding gas bubbles speeds the process up substantially.

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This is why proper aquarium aeration is essential to a healthy ecosystem. If there isn’t enough action at and near the surface, oxygen and carbon dioxide may not exchange fast enough and can cause your fish to suffocate.

Swift-flowing mountain streams and immediately downstream of waterfalls are the regions with the highest levels of dissolved oxygen as a result.

Air Breathing Fish

The vast majority of fish breathe water only and will quickly suffocate when taken out of the water. Their gills are fine structures for gas exchange in the water but are mostly non-functional in the air. A few fish can use their gills in the air for a while if kept moist but the majority will die in minutes.

However a few fish have adaptations specifically for breathing air. The most common are modifications to the gills, swim bladder, and true lungs!

Bettas and Gouramis have a specialized structure called the labyrinth organ. Located in front of the true gills, these modified gills allow the fish to take in atmospheric air. When kept moist they can survive for hours and are often sold packed in wet grass in Asian fish markets. However they evolved this ability to survive in the hot, still ponds and canals of Southeast Asia where oxygen levels can plummet and the majority of fish can’t survive.

Swim bladder modifications may be the most common way fish avoid drowning. Saltwater Tarpon and freshwater Arowanas are two fish that use this strategy. The swim bladder is a gaseous balloon that helps keep fish buoyant in the water. Many fish can inhale air directly into the swim bladder, which is lined with blood vessels to act as a primitive lung.

Fish with lungs are rare, much like labyrinth fish. Lungfish you’ve probably heard of. In the aquarium trade Bichirs and Reedfish are probably the most common fish with lungs around. Each of these fish can inhale and exhale if there isn’t enough oxygen in the water for their gills and their lungs have complex inner structures much like ours to increase gas exchange.

Since their natural habitats tend to be very low in oxygen these fish can actually drown if they can’t reach the surface every 30 minutes or so!

Warm Water

Water that’s too warm is another common way that fish can drown. “But wait,” you might be thinking. “These are tropical fish!” That’s true and as tropical fish, they do require warm water. However, warm water holds less oxygen than cold water, making proper aeration and turnover even more important the warmer the aquarium becomes.

Still ponds in very warm climates have little fish life. In the summer in temperate and warm countries, you may see fish swimming near the surface continually. Here the oxygen levels are the greatest if the water isn’t turning over properly.

Near the bottom oxygen levels are so low that fish can’t survive for very long. However once things cool down the water can hold more oxygen and fish return to the deeper parts of the pond.

The symptoms of water that’s too warm are pretty similar to water that’s too still for proper gas exchange. Typically both conditions exist at the same time in an oxygen-poor environment. You’ll see your fish rapidly breathing – literally gasping for oxygen.

They will hover near the surface, using as little energy as possible. They may dash deeper if afraid but they will begin to suffocate and return to the surface once the danger is gone.

This means your oxygen levels are dangerously low and you should address it by lowering temperatures and/or increasing turnover for proper gas exchange!


Conclusion

While fish rarely drown it’s still very possible. The fastest way is to place them in the air. While there is far more oxygen, fish simply can’t access it and they will eventually drown in the air.

However, most fish are in danger of suffocation if water conditions grow too poor for them to survive. Bettas and other air-breathers evolved mechanisms for dealing with this, but you should still provide them aeration, filters, and other technology to keep them healthy and stress-free!

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