Iron sulphate and polyaluminum chloride are commonly used in water restoration to eliminate cyanobacteria bloom and improve water quality. Nevertheless, the influence of coagulants on water organisms remains insufficiently studied. The study involves the analysis of phosphate coagulants’ impact on calcium and magnesium concentrations in the Chara hispida community. The experiments were carried out in field mesocosms. Both coagulants were applied once in three different doses: 50.0, 100.0, and 200.0 cm³·m^-3. The application of coagulants caused a decrease of pH and calcium carbonate and magnesium carbonate dissolution. Although the changes were proportional to the coagulant concentrations, the aluminum coagulant triggered more considerable disturbances. The highest dose of iron sulphate caused the precipitation of hardly soluble calcium sulphate and the elimination of part of calcium from biological circulation. The concentrations of magnesium in water increased only at pH <4.5 following the application of the highest dose of polyaluminum chloride. Shifts in the Ca-Mg equilibrium, which result in the disturbance of biogenic calcification, may affect charophyte metabolism and lead to the elimination of charophyte communities. Therefore, inactivation treatments using acidic coagulants in lakes with charophyte communities ought to be preceded by preliminary studies in order to determine the least harmful dosage for the ecosystem.