Brackish Water Aquariums
An introduction to brackish aquariums: salinity and specific gravity ranges, mixing marine salt, buffering and suitable species such as mollies, puffers and gobies.
What brackish water is
Brackish water has more salt than fresh water but less than the sea. By definition it holds between 0.5 and 30 grams of salt per litre, expressed as 0.5 to 30 parts per thousand, which corresponds to a specific gravity between about 1.0004 and 1.0226. It is not a single fixed condition, since natural estuaries vary in salinity over space and time.
Specific gravity ranges
Most brackish aquariums are kept at the lower end of this range. Reported working values include around 1.002 to 1.003 for mollies and bumblebee gobies, and about 1.005 for many brackish puffers and goby setups. The exact target depends on the species being kept.
Mixing salt
Brackish conditions are created with marine aquarium salt rather than ordinary table salt. As an example, dosing roughly 4 to 6 grams of marine salt per litre gives a specific gravity of about 1.002 to 1.003 at 25 °C (77 °F). The salt is dissolved fully before being added, and the result is measured with a hydrometer or refractometer.
Buffering and hardness
Brackish water is typically hard and alkaline. A goby setup at a specific gravity of 1.005 was reported at GH 22 and pH 8.1. Calcareous substrates such as coral sand or aragonite help maintain a stable, alkaline pH suited to these fish.
Suitable species
- Mollies (Poecilia), which do well in lightly brackish water
- Brackish puffers, which need a specific gravity of at least 1.005
- Bumblebee gobies and knight gobies
- Other estuarine fish such as monos, scats and archerfish in larger systems
Stocking note
Aquarium-strain mollies are usually hybrids that thrive in plain fresh water, while some wild molly species such as Poecilia gillii and Poecilia vandepolli prefer brackish conditions. Choosing species with matching salinity needs keeps the system stable.