<p>Microcystins produced by cyanobacteria have toxic effects on many species of aquatic and terrestrial animals, including humans. Bivalves are capable of accumulating large amounts of toxins in various tissues of their body. It is unclear whether cyanotoxins affect the health of bivalves. The research investigated how cyanobacterial toxins influence the ability of quagga mussels to maintain the balance of sodium, potassium, calcium, and magnesium ions between their bodies and the environment. During a 2-week experiment, a control group of molluscs was fed with <i>Chlorella</i> every other day, whereas the experimental group was fed both with cyanobacteria collected from the river and <i>Chlorella</i>. In the experimental group, the level of potassium and magnesium in the water in the state of ionic balance (C_balance) between the body and the environment was significantly lower than that in the control group. The coefficient of variation, the range min–max, and the range value for C_balance for the sodium, potassium, and magnesium ions in the water were also significantly lower in the experimental group. These findings suggest that cyanobacterial toxins may reduce cell membrane permeability, thereby reducing the rate of ion loss from the body into water and the rate of ion compensation through active transport from water. As a result, molluscs’ ability to maintain ion balance between their body and the environment is increased at lower and more stable ion concentrations in the water. This helps to increase the survival rate of molluscs in low-mineralized waters. The obtained results show that bivalves could be used to purify water from cyanobacteria and cyanotoxins.</p>