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Can Freshwater Fish Live In Saltwater? Answer & Exceptions

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Freshwater and saltwater home aquariums both house different beautiful species of fish. Since both are so unique, you may be wanting to mix freshwater and saltwater fish to get the best of both worlds. 

Freshwater fish can not live in salt water. If freshwater fish are put into a saltwater tank they will unfortunately shrivel due to dehydration. The water from the cells of the freshwater fish will leave the body- called osmosis. The exception is that euryhaline fish are able to be put into both.

So what is the science behind not adding freshwater fish to a saltwater tank and what are euryhaline fish? Read on to find out more! 

What Happens When Freshwater Fish Are Placed in Saltwater?

As stated, when a freshwater fish is placed in saltwater, they will lose the water in their bodies because of the hypertonic (salty) environment, which is called osmosis. With that being said, they will eventually die because of dehydration.

While this will not happen right when you put them in the tank, it does happen quite quickly. Most freshwater fish will only be able to live for a few minutes when out in the tank. However, if it was a mistake that was caught right away, they should recover quickly after being placed back onto a freshwater tank. 

If they are left in the saltwater tank, it will only take about a few minutes for them to die due to lack of water in their bodies and lack of oxygen. You may also notice that fish’s gills will start to swell, and they will also have a hard time keeping their temperature regulated.

With that being said, there are a few species of fish that are able to live in both freshwater and saltwater aquariums. These exceptions are called euryhaline fish. 

Group of healthy and active bleeding heart tetra adult fish

The Exceptions

Euryhaline fish are able to live in freshwater, saltwater, or even brackish water (a mixture of fresh and saltwater). There are not many species under this list, and said list only constitutes about 3-5% of all fish species. The list below are some aquarium fish that are considered euryhaline fish:

  • Short-finned mollies
  • Danios
  • Moonyfishes
  • Spanish toothcarp
  • Pupfish
  • Mayan cichlids

Out of these smaller fish listed, the most common in households is the short-finned molly. They come in a variety of colors and can be a great addition to any tank. 

The Science Behind Euryhaline Fish

So how can the above fish inhabit both freshwater and saltwater aquariums? The answer is that they are able to osmoregulate. Osmoregulation is the process where a fish is able to regulate its own body’s water content. This prevents the fish from dehydrating when placed in salt water. There are also two different types of osmoregulation.

Osmoconformers

Osmoconformers are euryhaline fish that are able to match their bodies’ osmolarity to the environment that they are in. This means that they control their salt concentrations to match their surroundings.

Aside from a few aquarium fish, most sea invertebrates are also osmoconformers; examples include seagrass, sharks, and marine mammals. 

Osmoregulators 

Regardless of the environment, most  fish use osmoregulation to fight the processes of diffusion and osmosis. This allows them to maintain the internal balance of salt and water essential to their efficiency and survival.

What happens if you put saltwater fish in freshwater? 

As we’ve established, if you put freshwater fish in saltwater, the freshwater fish will become so dehydrated that they will shrivel up and ultimately die within a few minutes. However, the opposite is true if you put saltwater fish in freshwater. 

If this happens, the water will go into the fish’s body until their cells retain so much water that they bloat. Because of the excessive bloating, the fish will eventually pass away. 

Tap Water: Is it safe?

Speaking of freshwater and saltwater, many people are curious about whether you can use tap water in aquariums. The answer is yes, but it would need to be treated. Most tap water has chlorine added to it, which is extremely toxic to fish. 

This means that to be usable for a freshwater or saltwater aquarium, all of the chlorine must be removed to prevent any fish from dying. There are several solutions available for tanks that dechlorinate the water at local pet stores. As always, with any aquarium solution, you should follow the bottle directly and not deviate from the amount of product to put into your size tank. 

Once the solution is mixed with the tap water it is safe to put into the aquarium. This makes for the perfect, safe freshwater for aquariums.

How to Make Saltwater for Aquariums 

Reef salt is commonly used with tap water to create saltwater for home aquariums. It is very important to follow the directions on the bag of reef salt to ensure there is not too much salt in the water. The following list is the steps needed to make proper saltwater:

  1. Purchase high quality reef salt. The higher the quality the better.
  1. Measure ½ cup of reef salt per each gallon of water you need for your tank. If you are doing a fill water change, you will want a few buckets to keep track of how many gallons you need to fill the tank. 
  1. After placing the salt, add one gallon of fresh water to the bucket for each ½ cup of sea salt. For example, you would need 4 gallons of water to dilute 2 cups of salt mix. 
  1. Aerate the water with air pumps. Using an air pump instead of stirring, will cause the salt to completely dissolve in your buckets. If you have a large tank, you may want to get a few air pumps so the process goes a bit quicker. 
  1. After the mixture is properly mixed, you will want to test the pH with testing strips to make sure it is the right parameters for your tank. 
  1. The final step is to use a hydrometer to measure the specific gravity. Specific gravity is the ratio of the density of saltwater relative to the density of the same volume of pure water. For most reef aquariums, the hydrometer should read about 1.025. 

Salt Can Treat Diseases of Freshwater Fish 

Yes, you read that right! Salt can be used to treat certain diseases common in freshwater fish. But, didn’t we say freshwater fish can’t survive in saltwater? The answer to that is also yes! The difference is what kind of salt you use. 

Aquarium salt is best for treating diseases. This is not reef salt, and shouldn’t be confused with saltwater tanks. There are different components to aquarium salt that make it safe to use while diluted. 

With that being said, a common treatment for fish is to dilute 1 tablespoon per every 3 gallons of water. This mixture is one of the best remedies for fungal infections, bacterial infections, and external parasites. 

It is important to note that you are not putting these gallons in the aquarium. Instead, you will take the infected fish out of the tank and put them in the diluted gallon of water and salt. It is recommended to keep them in the solution for about 30 minutes. 

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