Scat, Scatophagus argus
Estuaries, salt marshes, river deltas and mangroves are places where fresh water meets salt water and because of this brackish water contains more salt than fresh water but less salt than sea water. So they are an in between zone and a place where many fish species choose to live or breed. The species of fish which live in these areas are quite specialised and they will only really thrive in captivity when kept in the right conditions. To complicate matters more, some species of fish require different concentrations of salt in their water at their different life stages.
Water is classed as being brackish if it contains more than 0.5 grams of salt but less than 30 grams of salt per litre
Many species of fishes which are often sold as fresh water species actually live in brackish water in the wild. If you are setting up a brackish water aquarium there is probably a wider choice of fish than you may have initially thought. The list below gives some examples of commonly available brackish species although the list is by no means complete.
At this point it is worth mentioning that some fish are commonly treated as brackish water species when in fact they are only found in fresh water. All but three species of spiny eels are fresh water fish and the brackish species live at the very low end of salt concentration.
Few plants will thrive in brackish water but one or two will tolerate it.
Most importantly you should use marine salt when preparing brackish water because it contains all the trace elements and buffers that other salts lack. The concentration of the salt can vary from species to species but if you aim for an SG between 1.005 to 1.010 as this will suit 99% of brackish fish species.
Lighting, filtration, heating and cycling are exactly the same as with any other aquaria but brackish water holds less free oxygen than clean fresh water at the same temperature and this will have to taken in to account when it comes to stocking levels.
With the fish removed work out how much salt is needed to reach the desired specific gravity and then mix that salt with a bucket of tepid water until the mix is clear with no sign of milkiness (undissolved salt) and then add it to the aquarium and allow it to mix. Leave the filter running, the filter bacteria in most brackish water is exactly the same bacteria that is found in fresh water so after a week or two the filter will work as before without the need to recycle the tank. Only when the brackish water is close to sea water will the tank need a little time for a new bacterial colony to grow.
Water with salt holds less free oxygen than clean fresh water making it more important to keep up with good tank maintenance and keep a few less fish.