Effect of Water pH and Hardness on Survival and Growth of Freshwater Teleosts

The freshwater contains a variable amount of dissolved substances, depending on the soil in which it occurs. Most teleosts species survive to acute pH changes down to water pH 4.0–5.0 or up to 9.0–10.0, but exposure to more acidic or alkaline waters is lethal within a few hours. Mortality of fishes exposed to acidic soft water seems related to a decrease of around 50% of plasma ions, mainly Na⁺ and Cl^-. The protective effect of water hardness against low pH is dependent of the affinity of branchial tight junctions for Ca²⁺. Water hardening of fertilized eggs varies according to species and water hardness. Most teleosts exposed to acidic or alkaline waters showed a higher survival in hard rather than in soft waters. Additional studies regarding the growth of fish whose natural environment is acidic or alkaline are lacking.