What is the Specific Gravity of Brakish water. Is there a safe varience?

Answer 1

Rather than a specific value, brackish is better described as a "range" from 1.006 to 1.015. This environment can occur either within inland areas with higher mineral contents where the salinity is determined by rain, runoff, and outflow or in costal areas where freshwater (rivers & lakes) can be intruded upon by saltwater as a result of tides, hurricanes, and changes in groundwater levels. As you might guess from this information, the exact specific gravity for fish in these areas can be quite variable. It can, and does change frequently. Adding to that, some of the fish that are considered "brackish" move between salt and freshwater at times of the year for spawning.

What's "safe" really is determined by the individual species. Guppies and mollies, although these are considered "freshwater" fish my most aquarists, are really brackish, and can live in full freshwater to full saltwater - I've kept them in both environments myself. Others (some of the puffers) prefer narrower ranges, and need to have increased salinity as they mature. Your best option here it to research any species in which you're particularly interested to see what conditions each perfers, and only mix those with similar requirements. I'll post a link below where you can find information of some of these species, but you may also want to do a little research of your own and get advice from a number of sources.

If you're interested in fiddler or red-claw crabs, which also require brackish conditions, I'll post some links to their needs as well.

Answer 2

brackish starts at 1.002 on a hydrometer.