They do not have an ability to stop water from diffusing into them ,so long exposure to less than 1.008 will kill them. Long term exposure to sg lower than their natural environment will likely play out on the kidneys overtime but makes them work less in the short term.

How low salinity can fish survive?

Salinity refers to the salt content of water. It is often measured in terms of specific gravity, or the amount of dissolved salt. In general, fish thrive in an intermediate specific gravity of 1.023 to 1.025.

Is low salinity bad for fish?

Lower salinity makes it harder for most parasites to thrive, and lowers stress for fish.