Brackish water fishes If brackish water fish are being kept, marine aquarium salt should be used rather than aquarium salt. The precise salinity isn't usually important, with anything from 5 ppt (about 0.6 oz/gallon) through to 18 ppt (2.4 oz/gallon) being recommended. The low end of that range corresponds to about 15% normal seawater salinity and is ideal for brackish water communities based around livebearers, gobies and plants. Higher salinities may prevent the use of plants but will be better suited to the big brackish water species such as monos, scats, archerfish and Colombian shark catfish.