What is brackish water?

What is brackish water?

Brackish water, also known as brack water, is less salty than seawater but more salty than freshwater. As long as the water salinity (based on dissolved salts) is 0.5 ppt or less, the water is considered freshwater. If the salinity is 30 ppt or more, the water is considered saline. Brackish water falls in between; from 0.5 ppt to 29 ppt.

Brackish species

There are many species of fish, invertebrates and plants suitable for brackish aquariums since brackish water occur in many different habitats and climate zones all over the world. Brackish water can for instance be the result of a freshwater river emptying itself into the ocean, and there are also brackish seas and lakes, e.g. the Baltic Sea and the Caspian Sea.

Some species live their entire life in brackish water, while others spend their youth in brackish water before migrating into the ocean. This means that you have to research the species you are interested in keeping, because you might have to change the salt content in the aquarium as they mature. Some species of fish migrates through brackish water to breed; the salmons will for instance leave the ocean and swim upstream to breed, while some species of eel live in rivers and streams but return to the sea to breed. Highly migratory species are however rarely a good choice for the aquarium since it is impossible for them to carry out their natural behaviour in captivity. Last but not least, many aquatic creatures can move back and forth between saline and brackish water as they please, and some will even enter freshwater. The Bull shark (Carcharhinus leucas) can for instance be encountered freshwater, brackish water and marine water.

Fish for brackish aquariums

If you want to set up a brackish aquarium, there are many different species to choose among and even beginner aquarists can find suitable inhabitants for a brackish aquarium. Some popular beginner species sold as freshwater fish will actually do even better in brackish conditions, e.g. the Black molly (Poecilia sphenops). Two other examples of fish species suitable for a beginner aquarist with a brackish aquarium are the Sailfin molly (Poecilia latipinna) and the American flagfish (Jordanella floridae).

If you have some more experience with fish keeping you can chose moderately difficult species for your brackish aquarium, such as Banded archer fish (Toxotes jaculatrix), Bumblebee goby (Brachygobius doriae), Fahaka puffer (Tetraodon lineatus), Figure 8 pufferfish (Tetraodon biocellatus), Mono argentus (Monodactylus argenteus), Mono sebae (Monodactylus sebae), and Scat (Scatophagus argus).

Plants for brackish aquariums

If you want to keep your brackish aquarium planted you have to choose species with care, especially if your brackish aquarium is closer to marine salinity than to freshwater. Plants occur in brackish environments in the wild and such species are naturally a good idea for a brackish aquarium, provided of course that you can satisfy their other requirements. It should however be noted that quite a few of the really common plants offered by fish stores as freshwater plants can be adapted to brackish conditions if you let the change be gradual. Plant them in a freshwater tank and give them a chance to establish themselves before you start increasing the salinity step by step. Many suitable plants can be found in the genera Anubias and Elodea, and in the division Anthocerotophyta (Hornwort). Java fern (Microsorum pteropus) and Water sprite (Ceratopteris thalictroides) are also known to work well in brackish aquariums.

Equipment etcetera

First and foremost, you will need salt – real aquarium salt suitable for marine aquariums. When it comes to equipment, a brackish aquarium is very similar to a freshwater aquarium but you need to purchase a hydrometer to measure the salinity of the water. Simply adding a pinch of salt now and then will not cut it; you need to know what you’re doing. Water evaporates from an aquarium and makes it virtually impossible to calculate the exact salinity. When you have gained more experience, it may be possible to keep estuarine species without using a hydrometer since estuarine species are used to constantly varying salinity levels. Many aquarists deliberately change the salinity back and forth to more accurately mimic the natural environment of their estuarine fish.