Brackish/ˈbrækɪʃ/ Definition 1 of 2Pro Tip 1/3Natural Habitats Often used to describe water in environments where sea and fresh water mix, like estuaries. Mangroves are typically found in brackish waters. Pro Tip 2/3Aquarium Care Some fish are kept in brackish aquariums; know this if discussing fishkeeping. The figure 8 puffer thrives in a brackish aquarium setting. Pro Tip 3/3Not Freshwater Remember, 'brackish' implies that the water isn't suitable for drinking or agriculture. Despite the rains, the well remained brackish and undrinkable.