Milligram per L is a unit of concentration often used for chemicals in aquaria. For chemical concentrations in freshwater, 1 mg/L is approximately 1 ppm (= 1 mg/kg). For chemical concentrations in seawater, where 1 L weighs approximately 1.023 kg, 1 mg/L is approximately 0.978 ppm (= 0.978 mg/kg).