Goldfish are the most commonly owned freshwater fish in the world. Modern goldfish are a domesticated version of wild carp that originated out of East Asian rivers. And they can be among the hardiest fish you’ll come across.
They can survive in bodies of water with low temperatures and little oxygen. Some species of goldfish even thrive in outdoor ponds instead of indoor aquariums such as Koi, Rudd, Tench, and Orfe.
But can these freshwater fish survive in brackish conditions?
Recent studies have shown that goldfish can indeed survive brackish environments—just not too salty. And while this may seem like a win for the goldfish, there’s actually a big underlying issue with it.
What Is Brackish Water?
Brackish water is an environment that’s halfway between freshwater and saltwater. It’s not too salty, but it’s not necessarily devoid of salinity either. Brackish water doesn’t have a clear-cut value. It can cover a wider array of ranges.
Brackish water is often described in terms of parts per thousand (ppt) salinity. The ocean has a 35 ppt salinity rating and anything less than that can technically be considered brackish.
There are several species of fish that can absolutely thrive in brackish environments. Goldfish are not generally considered anything but freshwater fish. This means they should have a zero ppt rating for their aquarium. However, as you’ll see, that doesn’t mean that they can’t leave their comfort zone.
Goldfish in Brackish Water
There shouldn’t be too much surprise among goldfish enthusiasts regarding a goldfish’s ability to withstand brackish water. As a matter of fact, goldfish fanciers have been adding small amounts of sea salt (or aquarium salt) into their tanks for years to help keep them healthy. But just how much salt can goldfish handle?
According to a study performed by James Tweedley and others from the Centre of Fish and Fisheries at Murdoch University, goldfish have been found in estuaries much saltier than your tank at home. They found goldfish within the Vasse and Wonnerup estuaries with salinities registering at 17 ppt salinity. This is halfway between a standard aquarium tank and the ocean itself.
And this isn’t an isolated incident either. In fact, 526 different goldfish were found in the same waters throughout just three surveys. Some of these fish were quite massive as well, weighing in at around 4.5 pounds. This makes these brackish goldfish some of the biggest in the world!
However, that’s not necessarily a good thing. These goldfish have become an invasive species to the waters they’re now swimming in. With the bountiful selection of food and the massive size of the estuaries, these fish are rapidly growing to a size in which they have no natural predators to cull the population.
These monster-sized brackish water goldfish have been wreaking havoc within the estuaries’ ecosystems by accelerating algae blooms, disturbing sediment, and consuming the eggs and spawn of those fish native to the area.
And they’re spawning.
This can be extremely detrimental to the current ecosystems, but this isn’t even the scariest part. What could make matters worse is if the goldfish can use the estuaries as a “salt bridge” and spread them to other freshwater rivers leading into them. This would perpetuate their invasion through other ecosystems.
Can I Keep Goldfish in a Brackish Aquarium?
Other studies have been done on the effects of brackish water on goldfish with surprising results. In a study by Semra Kucuk of the Adnan Menderes University in Turkey, goldfish can indeed survive without negative consequence in brackish waters—as long as the salinity does not exceed 8 ppt salinity. The study also shows that the maximum salinity a goldfish can withstand is 20 ppt salinity.
Most brackish aquariums run between 9-19 ppt salinity. However, it’s on the lower end of that spectrum where you’ll see true brackish fish species thrive. So, if you were to lower that spectrum by just one part per thousand, you’d be able to comfortably accommodate most true brackish fish with your freshwater goldfish.
Should You Keep Your Goldfish in Brackish Water?
Both of the independent studies we mentioned above show that goldfish can thrive in brackish waters. However, there is still much to be learned. We know for a fact goldfish can live happily and for long lives in cool, fresh water. And until more information comes along to further solidify these findings, you should probably keep them in freshwater tanks.
Featured Image Credit: Glyn Baker, Wikimedia Commons