The main objective of the present study is to determine lead toxicity on various sizes of zebra fish in soft and hard water. Lead toxicity analysis was carried out in the laboratory trials for four days at various concentrations for the determination of LC50 value. Zebra fish was grouped into various sizes (G1-G4) and treated with lead for 96 h. Lead induced toxicity and induced various behavioural changes in Zebra fish. In the case of G1 (1.5 ± 0.2 cm length) fish the LC50 values ranged from 27.2 ± 1.8 mg/L to 9.7 ± 1.3 mg/L. In G2 (2.0 ± 0.2 cm length) fish, the LC50 value was 37.01 ± 2.3 mg/L and it reduced as 32.03 ± 3.4 mg/L after 48 h. In G3 group (2.5 ± 0.2 cm length fish), LC50 value was 38.78 ± 2.4 mg/L after 24 h treatment and this value decreased as 33.18 ± 1.1 mg/L after 48 h. After 72 h, LC50 value was 27.2 ± 2.1 mg/L and this value decreased considerably after 96 h (18.3 ± 2.1 mg/L) in G4 (3.0 ± 0.2 cm length) fish. Further, G4 fish was treated with lead in hard water and LC50 value was analyzed. LC50 value was observed as 28.62 ± 2.7 mg/L in hard water and the same fish group (G4) showed 18.3 ± 2.1 mg/L after 96 h. Toxicity analysis revealed that lead is less toxic in hard water than in soft water.