Salinity is a key parameter for aquarium health. Different tank types require different baseline salinity levels to keep inhabitants comfortable and thriving.

Freshwater tanks typically have near-zero salinity, measured as 0 ppt (practical salinity units) or a specific gravity close to 1.000. In practice, most hobbyists monitor for very slight chlorinated or dissolved mineral content rather than a true salt concentration.

Brackish tanks sit between freshwater and saltwater, usually in the range of about 0.5–1.0 ppt (roughly SG 1.005–1.010). These setups house fish and invertebrates adapted to intermediate salinity levels, so accuracy and consistency are especially important.