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Brackish aquariums

Brackish Water Aquariums

The Fascinating Space Between Fresh and Salt

Brackish aquarium with archerfish, puffer, mangrove roots, and salinity meter

Introduction

Brackish water occupies the ecological space between freshwater and the ocean: estuaries, mangrove swamps, tidal rivers, and coastal lagoons where fresh river water and salt ocean water mix in constantly shifting proportions. It is one of the most productive and biologically rich environments on earth, and it is almost completely overlooked in the aquarium hobby.

Brackish aquariums are not as difficult to maintain as marine tanks, and many of the most interesting and characterful fish in the hobby are brackish species that spend their entire lives caught between two worlds. If you are looking for something different from the standard community tank, brackish is worth serious consideration.

Quick Overview

Water typeBetween freshwater and marine conditions
Measured asSpecific gravity, usually around 1.003-1.015
UseMarine salt mix, not plain aquarium salt
Popular fishFigure-eight puffers, gobies, archerfish, mudskippers, and mollies

Understanding Salinity

Salinity in brackish tanks is measured as specific gravity (SG) using a hydrometer or refractometer. Pure freshwater has an SG of 1.000; full marine water sits around 1.025. Brackish tanks typically fall between 1.003 and 1.015 depending on the species being kept.

  • Low-end brackish (SG 1.003-1.006): suitable for figure-eight puffers, mollies, some gobies, and archerfish; resembles tidal estuaries
  • Mid-range brackish (SG 1.008-1.012): appropriate for mudskippers, green spotted puffers (juveniles), and many goby species
  • High-end brackish (SG 1.015-1.020): preferred by adult green spotted puffers and some scats; approaches marine conditions

Use marine salt (reef salt or standard marine salt mix), not aquarium salt (NaCl only), for brackish tanks. Marine salt contains the full spectrum of minerals found in natural seawater, which brackish fish require for proper osmoregulation.

Popular Brackish Fish

Figure-Eight Puffer (Dichotomyctere ocellatus)

The figure-eight puffer is the most popular brackish fish in the hobby, and deservedly so. It is small (2-3 inches), strikingly patterned with yellow and green on a dark body, and has an enormous personality packed into a tiny frame. It is intelligent, curious, and recognizes its keeper. It requires live or frozen shellfish foods to wear down its constantly growing beak.

Green Spotted Puffer (Dichotomyctere nigroviridis)

Larger than the figure-eight (up to 6 inches), more aggressive, and requiring higher salinity as it matures. Green spotted puffers need a species-only tank or very carefully chosen companions, as they will bite fins and attack tankmates. Full adults do best in near-marine conditions.

Archerfish (Toxotes species)

Archerfish are famous for shooting jets of water at insects above the surface to knock them into the water. In a brackish tank with an open top, they will demonstrate this behavior at feeding time. They grow to 6-10 inches and need a large tank with several inches of air space between the water surface and the lid.

Mudskippers (Periophthalmus species)

Mudskippers are among the most extraordinary fish you can keep: amphibious gobies that spend much of their time out of the water, breathing through their skin when moist. They require a paludarium-style setup with a large land area, warm ambient air, and high humidity. Fascinating and completely unlike anything else in the freshwater hobby.

Mollies

Mollies are unique among common livebearer species in their true tolerance for brackish and even full saltwater. A brackish tank with low salinity (SG 1.003-1.005) keeps mollies in exceptional health, with more vibrant color, increased disease resistance, and reduced susceptibility to the fungal infections that plague mollies in pure freshwater.

Plants in Brackish Tanks

Most freshwater aquatic plants do not tolerate salt and will decline or die in brackish conditions. Brackish tanks are typically decorated with hardscape rather than live plants, though a few species can tolerate low-end brackish conditions.

  • Java fern tolerates mild brackish (SG up to about 1.005)
  • Java moss has some brackish tolerance at low salinity
  • Mangrove propagules can be grown with their roots submerged and leaves above water, providing authentic decor
  • For higher-salinity setups, focus on rock and driftwood hardscape rather than plants