A test system for determining the dissolved oxygen content in water in the range of 0 to 14 mg/L is developed which consists of a tablet, an ampule, and an indicator scale. Testing is carried out on eight water bodies in the Yadrinsky District of the Chuvash Republic: the Sura, Vyla, Arbashka, and Mochkaushka rivers; Sergach and Round Pit lakes; and Mirsky Pond. Samples were taken in June, 2024 in accordance with GOST (State Standard) 59024-2020. The reliability of the results is confirmed by the Winkler iodometric titration method. It is found that the dissolved oxygen content in the water samples from the considered reservoirs is at a level of at least 6 mg/L, which corresponds to the norms of RD (Regulatory Document) 52.24.419-2019 for fishery facilities in the summer season.