What counts as brackish? How much salt before water becomes brackish instead of fresh? All freshwaters have some amount of salts, some have more than others. People who keep fish often add significant salt to the water, are these then brackish? The hermit crab mentioned can live and often chooses to live far up stream from the marine environment. A salinity of about 1.004 is enough to maintain them indefinitely. Most if not all freshwater fish will do quite well at salinities of that level.