If you are setting up an aquarium in your home, you might be wondering about stocking your tank and whether to create a saltwater tank or a freshwater tank. You might also be wondering whether you can mix and match the different species, or if this will cause problems.
You can sometimes put saltwater fish in freshwater, but on the whole, fish have a narrow range for their salinity tolerance. Most fish will die if you move them from one kind of water to the other, especially abruptly. There are some species of fish that can survive in both freshwater and saltwater, including things like barramundi, Atlantic stingrays, and American eels.
In this article, we’re going to explore why fish struggle with changes in salinity, which fish cope better than others, and everything else you should know about saltwater and freshwater fish and their needs. This will help you to understand how fish survive.
Why Can Saltwater Fish Live In Freshwater?
Saltwater fish can’t live in freshwater in most cases. Their bodies are adapted for survival in saltwater, and if you move one from saltwater to freshwater, it will almost certainly die in a short space of time. It may also show signs of stress and disease.
Some fish are able to adapt better to changes in the salinity levels, and these are known as Euryhaline fish. Many of these fish swap between a saltwater and a freshwater environment at some point during their lives, so they have the ability to adapt to changes in the salinity. However, most fish do not have this ability.
Unfortunately, moving a saltwater fish to freshwater will almost always result in it dying. This is because the salt concentration within the fish’s body is much greater than the salt concentration in the surrounding water. This will cause a constant flow of water into the fish’s body, because the salt will pull the water inward.
Saltwater fish depend upon a process called osmosis to survive. Osmosis determines how much water moves between the fish’s cells and the external environment. Hypertonic cells allow a saltwater fish to absorb water without losing salt – but they can’t do this in a freshwater environment.
To survive in freshwater, fish would need hypotonic cells. If you put a saltwater fish into freshwater, it will be unable to access the salt that it needs to regulate its cells. The salt stored inside it will start to transfer to the surrounding water, and the surrounding water will flow in.
The fish will start to swell up and become bloated, and this will kill it.
What Saltwater Fish Can Live In Freshwater?
There are a few saltwater fish that have the ability to adapt to freshwater, although these are relatively rare. They are known as euryhaline fish. The list includes things like:
- Bull shark
- Mangrove Jack
- Striped bass
- Atlantic stingray
- White Perch
Many of these have evolved this ability because they spawn in a different kind of water to the one they usually live in. For example, the barramundi lives in freshwater for the most part, but then swims downstream to the sea when the time comes for it to spawn.
The eggs of this fish require saltwater or brackish water in order to develop. They are usually found in tidal flats and estuaries, and they will hatch and grow here. When the fry are ready, they will swim upstream to the freshwater where the adults reside.
This is seen in other fish species too, sometimes in the reverse. Some kinds of fish swim upstream to spawn, and the young swim down to the sea once they are large enough. The North Atlantic salmon would be one example of this.
However, these fish often need time to adjust, and the chemistry of their body will need to change in order to survive the salinity fluctuation.
Can Freshwater Fish Live In Saltwater?
No; freshwater fish cannot live in saltwater for the same reasons that saltwater fish cannot live in freshwater. The fish is not designed to survive in saltwater and will not be able to regulate the amount of water in its body either.
Freshwater fish will lose too much of the water from their bodies far too quickly if they are placed in saltwater. The salt will pull the water out of their cells, leaving them dehydrated. They will be unable to regulate the movement of the water as they can in freshwater, and so they are likely to die very fast.
As mentioned above, there are some fish that move from freshwater to saltwater, but they will have to take time to adapt and let their bodies change.
Can Freshwater Fish Survive In Saltwater?
Most freshwater fish cannot survive in saltwater, no. Those that can are called euryhaline fish, as mentioned above, and they might swim to the sea in order to breed and lay their eggs. The specific groups that do this are known as Anadromous fish. They will not remain in the saltwater for long, but will return to the rivers and lakes once their eggs are laid.
If you put a freshwater fish into saltwater, it will die quickly. Its tissues will become dehydrated and it may even visibly shrink as the salt pulls the water out of its body. It doesn’t have the necessary cells to prevent this from happening.
Even if you have a species that is capable of tolerating brackish or saltwater, you should give it plenty of time to adjust to the change in salinity.
Saltwater fish cannot live in freshwater and vice versa in most cases. The fish have evolved to survive in their specific setup, with cells that help to regulate the amount of water passing into and out of their bodies. If you want to keep fish yourself, you must choose either saltwater or freshwater fish, and set up your tank accordingly.