Brackish aquarium: which species and how to set it up
Brackish water, halfway between fresh and marine, matches the estuaries and mangroves where euryhaline fish live, able to withstand a wide salinity range. The molly (Poecilia) is the classic example: work comparing sailfin molly populations from fresh and brackish water showed their remarkable osmoregulatory abilities (research on Google Scholar). A brackish tank is neither a marine aquarium nor a freshwater tank randomly salted. Find our other pointers in the bedding and habitat section.
What salinity to aim for
Salinity is measured by density, with a hydrometer or refractometer. A beginner brackish tank generally sits around 1.005 to 1.010 density, well below the 1.020-1.025 of a marine tank. You prepare the water with aquarium marine salt, never table salt, dissolved in advance. Our guide to salt in a freshwater aquarium explains why dosing matters so much.
Which species to choose
Mollies, gobies (such as the bumblebee goby), archerfish, and some killifish and shrimp tolerate brackish well. You must gather species with the same salinity and temperature requirements, and avoid mixing strictly freshwater fish with brackish species. An overstocked or ill-matched tank remains the leading cause of failure, in brackish as anywhere.
Setting up the tank
A sandy substrate, mangrove roots, a few rocks and salt-tolerant plants (such as some hardy cryptocorynes) recreate an estuary look. Filtration must be solid and the tank properly cycled before stocking, exactly as in fresh water. If you are still weighing your options, our comparison freshwater or saltwater for beginners places brackish between the two.
Budget and gear
To the usual gear you add a density-measuring tool and marine salt, two modest but essential items. We detail the full envelope in our guide budget for a beginner brackish aquarium.
Frequently asked questions
Is table salt fine for a brackish tank?
No: use aquarium marine salt, formulated to supply the right minerals. Table salt, unsuited, throws off the water chemistry.
Can a beginner start with brackish water?
Yes, provided you measure density and choose easy euryhaline species like the molly. It is a good intermediate between fresh and marine water.
Can I keep plants in a brackish tank?
Some salt-tolerant species yes, but the choice is narrower than in fresh water. Mangrove roots and mineral decor make up for it well.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.