Two families of livebearers provide suitable inhabitants for brackish aquariums.  Family Poeciliidae includes the livebearers we are all familiar with (guppies, mollies, moons, and swordtails).  All are suitable for brackish aquaria.  Fancy guppies are not recommended for brackish communities, however, because they are just too delicate for the temperament of the average brackish fish, and may get rather badly beaten.

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Pineapple swordtails add color to brackish water tanks.

Swordtails and moons do just fine in brackish water, since all livebearers prefer a little salt in their water.  If you use marine salts in your tank, or use much over two teaspoons of regular salt in your water, you may want to adjust these fish to the new water slowly.  I don’t really recommend them in tanks with a salinity of much over 1.003.

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Mollies no longer come in just black and green.

Mollies, however, love salt – particularly marine salt – and do just fine in full-strength seawater (slowly adjusted over time).  They will actually do much better in brackish water than the freshwater most people keep them in.  They develop larger dorsal fins and prettier colors in brackish water.

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Females above.  Male below.
Couldn't quite catch the orange trim on his tail.

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Mollies look good -- especially the sailfins.

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Naturally the sailfins cost more.

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Gambusia are the outcasts of the livebearer world.

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Wrestling half-beak.

Halfbeaks.  The other livebearer family of major interest to brackish water aquarists is the family Hemirhamphidae, the halfbeaks.  The common halfbeak, Dermogenys pusillus, isn’t so common anymore.  It is not hardy, and rather picky as to eating habits, preferring live food.  Males also like to fight each other.  Also known as wrestling halfbeaks, they are often the subject of large wagers – like bettas.  They can be adjusted to flakes, but only high-quality foods will keep them alive long.  The Celebes halfbeak, Nomorhamphus sp., is larger and more colorful, with black-edged jaws and fins of healthy specimens tinged with blood red.  They are fairly hardy, but, again, only high-quality foods suffice.  This is an excellent surface dweller for a larger community tank.

Bull Shark.  There is only one truly suitable scavenger for the saline conditions of the brackish community, namely the so-called “bull shark”.

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Most bull sharks start out at about two inches.

Actually a member of the marine catfish family Ariidae, this fish Arius sp. is constantly active, often reaching the food at the surface before any other fish.  Not to worry; these fish have a great appetite, and still have room enough to clean uneaten food off the bottom when the food at the surface is gone (no excuse for overfeeding, however).  They grow fairly large; the largest one I have seen was just under 12 inches, and I feel certain that, given space, water quality, and food, they can grow much larger.  If yours gets too large for your tank, you can always trade it in.

Killifish Algae Eater.  In lieu of a suitable algae eater for brackish water aquaria, the diamond killifish, Adinia xenica, which is occasionally available, will help keep algae growth down.  This is a rather attractive, if not highly colorful fish that is quite peaceful, and will constantly pick at any algae it finds.

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(Killifish.  Most killies tolerate salt very well.  They mix well with the smaller brackish fishes, 
but are not as picky at meal time. LA)

There are many other species of fish that are suitable for brackish aquaria.  The availability of species varies.  If you are looking for something “out of the ordinary,” give brackish water fish a try!

Eric Crouch, Fish Consultant

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