Excessive algal growth poses serious environmental health problems, yet the development of a cost-effective solution for long-term inhibition of algal growth remains a huge challenge. This invention utilizes algicidal hydrogels comprising safe and environmentally friendly bioactive ingredients for practical applications in fresh water and sea water. The hydrogels can release oxidizing and cell-permeable algicides in a controlled manner to inhibit algal growth for a long time without adverse effects on aquatic organisms. Their controlled release performances and algicidal activities have been verified in laboratories and a 1500 m3 sea water reservoir in Hong Kong. Real-time monitoring equipment effectively provides data to adjust the amount of hydrogel and perform daily water quality testing.