Brackish fish

Author: Last updated:

Brackish fish are a relatively neglected group of aquarium fish yet they contain some of the most interesting fish in the hobby. For a biotope tank, a brackish setup is a good fish keeping challenge

Brackish tropical fisb - Knight Goby

At first glance it can seem that the choice of brackish fish is rather limited, but if you look a little deeper, you will find plenty of variety for any size of aquarium. The basic care principals are the same for brackish aquaria as they are for freshwater, the only extra knowledge you need is to understand the fish's natural environment, and that is exactly the kind of research that makes fish keeping such an interesting and rewarding pursuit.

Natural brackish areas

Brackish water occurs in nature wherever rivers reach the ocean and it is the mixing of these two bodies of fresh and salt water that produces the brackish conditions. Large river systems like the Amazon have brackish waters which may stretch for hundreds of miles inland and many miles out into the sea, providing a large habitat for brackish fish.

Along the course of this brackish area there may be many isolated pools and mangrove swamps. When the tides rise, seawater travels into the river and continues for many miles inland, increasing the waters salinity, whilst when the tide recedes, freshwater begins to dominate. Because saltwater is denser than freshwater the two do not mix immediately, and the less dense freshwater actually floats on top of the heavier saltwater.

The tides also cause a rise and fall in the rivers water level and combined with the rivers silt and debris build up, this creates vast expanses of nutrient rich mudflats which are exposed to the open air for half of the day. These nutrient rich areas contain a wealth of substrate dwelling creatures, which are a constant source of food for fish, birds and other aquatic animals.

Adapting to the environment

Most fish have the ability to either retain salts (freshwater fish) or excrete salts (marine fish) to keep the constant salt level required by the body to function properly. Brackish fish are designed to be able to carry out the functions of both freshwater and marine fish to a certain degree.

With some fish their ability to carry out either of these functions may be dependant on their age. Scats and monos for instance are perfectly fine in freshwater for the first half of their lives but as they age their bodies adjust to cope with saltwater, and if kept in freshwater they would become increasingly stressed and eventually die an early death.

This is one reason why fish from brackish waters must have some salt added to the aquarium, if they are kept in either completely fresh or full strength seawater, their bodies would eventually be unable to cope and they would die prematurely.

Salt levels

Salt can be measured in 'specific gravity' which measures the density of water based on the amount of mineral salts it contains, freshwater has a specific gravity of 1 whilst seawater is normally around 1.022-1.024

The easiest way to measure salt levels is with a hydrometer, a piece of kit which is vital to marine fish keepers. Hydrometers vary in design but the most common is simply a container with a needle inside which floats to a different level depending on the density of the water (heavier water means a more buoyant needle). After continual use, some hydrometers can begin to give inaccurate readings due to deposits on the needle so it is wise to replace hydrometers every year.

Because brackish waters vary in salinity it is difficult to provide a recommended salinity but anywhere between 1.008 - 1.015 should please most brackish fish. It is worth noting that most shops will keep their brackish fish in either freshwater or at a very low salt level.

When you set up a brackish tank it would be worth mimicking the shops conditions, even if it is pure freshwater, as this will reduce any stress during acclimatisation. Once the tank is stocked the salt level can be gradually increased.

Brackish fish in the aquarium

There are quite a few 'true' brackish fish and fish suited to brackish water available, in most cases these fish can be grouped into the larger, boisterous species and the more peaceful smaller species.

As with any fish, but especially in the case of some brackish species, it is important to find out as much as you can about the fish before you purchase any. The brackish environments in which these fish live may be quite different to the 'normal' freshwater habitats, which results in some brackish fish having specific needs. Some puffer fish for instance may need to be fed cockles still in their shells in order for the fish to wear down their teeth in the process of crunching open the cockles. Without this regular wearing down their teeth may become uncomfortably large and the fish will have trouble feeding.

Most brackish fish feed on high protein meaty foods such as cockles, mussels and shrimps in nature and will need the same kind of diet in the aquarium. The extra waste produced by these foods will require a good filtration system so an oversized external filter would be a wise addition.

Decor is relatively unimportant as most brackish fish live in quite open areas but hiding spots amongst pieces of root-like bog-wood would be welcomed. Bottom dwelling species such as shark catfish and gobies will need a sandy bottom to rummage through, or they could easily damage themselves on normal aquarium gravel.

Larger brackish fish

Probably the most popular and well-known brackish fish are the Scats (Scatophagus sp.) and Monos (Monodactylis sp.) of which there are a number of colour and pattern forms available. These fish are active and boisterous so can only be kept with other fish that can 'hold their own'. Keep these fish in groups of at least four or more; in small numbers one will become a dominant bully and attack weaker tank mates. Because these fish will easily grow to at least 15-20cm, a small group would require a large tank, at the very least 120cm long.

Another well-known fish, the Archerfish (Toxotes jaculatrix) is a surface dweller, which will also reach around 15-20cm although they do not need to be in groups. The Archerfish is so called because of its amazingly accurate ability to fire jets of water from the surface in order to knock insects of overhanging branches for food.

Another reasonably sized fish is the orange or silver chromide (Etroplus maculatus), a relatively peaceful cichlid that will mix well with medium to large fish. For the lower region of the aquarium a good scavenging catfish is the shark catfish (Arius seemanni) which sports a stunning silvery body that is enhanced by its constant movement. The shark catfish can potentially grow up to 30cm so should really only be kept in wide aquaria at least 150cm long.

Smaller brackish fish

There is a wider choice of small fish for the brackish aquarium and some naturally freshwater fish can also be included. Some of the freshwater species actually show better health if kept in slightly brackish water due to the difficulties that disease pathogens have coping with the extra salt, and the fishes reduced need to waste energy on retaining salts in the body.

For relatively low salinity brackish aquaria, popular livebearers such as mollies, platies and guppies are ideally suited. Other midwater swimmers include the glassfish (Chanda ranga), which is an interesting addition due to its see-through body. The glassfish needs to be kept in groups and make sure you avoid any shops that sell the cruelly died glassfish which sport fluorescent markings.

Some rainbow fish can be kept in mild brackish water although a little research will be needed to obtain the right species.

For the surface of the aquarium the halfbeak (Dermogenys pusillus) is an interesting fish and if you are lucky enough to find some for sale the four-eyes fish (Anableps anableps) is an intriguing aquarium addition. For the bottom of the aquarium either the knight goby (Stigmatogobius sadanundio) or the bumblebee goby (Brachygobius doriae) would be suitable.

Brackish fish are relatively easy to care for and certainly qualify as 'something different'. I have only skimmed the surface and mentioned some of the more aquarium suited fish here, but there are many more brackish species to consider, and far too many to fit into this article (puffers, mudskippers, tiger fish, shrimps, crabs etc.)

If you are a fish-keeper who likes a challenge you could try to create a brackish mangrove aquarium. In this style of aquarium it is possible to keep crab species, mudskippers and grow mangrove plants. Due to the need for crabs and mudskippers to burrow and be able to venture above the water, this style of aquaria is really best left to more experienced aquarists.

Click to read the Latest Tropical Fish discussions from Thinkfish users.

Think Fish Tropical Fish Forum
Tropical Fish Market Place
Fish of the month

Helping Fishkeepers With Their Fishkeeping Needs Since 2006