Raising fish in ponds for an extended period can have an impact on the accumulation of bacteria that can affect the health of the fish. Using plant extracts is an interesting option to consider for reducing bacterial contamination in ponds. This study investigated the effectiveness of cinnamon extract in reducing bacterial contamination in the water used for raising Siamese fighting fish (Betta splendens). Optimal concentrations of cinnamon extract were determined for microbial reduction in the water used for the cultivation of these fish. Test the ability to resist infection at the concentrations of cinnamon extract 1:1, 1:2, 1:4, 1:8, 1:16, and 1:64, against 3 bacterial species (Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa). The concentrations of cinnamon extract at 1:1 and 1:2 inhibited all three tested pathogens, while up to 1:4 inhibited S. aureus. Water samples taken from the ponds for testing were studied at 4 cinnamon extract concentrations: 0, 1, 2, and 3 gL-1. The treated water samples were collected at 0, 24, and 48 hours to quantify microbial activity. There were significant (p<0.05) differences among the concentrations, with the numbers of microorganisms in the treated water at 48 hours being 3.84x105, 2.92x104, 2.69x103, and 2.89x103 CFU mL-1, respectively. The experimental groups with concentrations of 2 and 3 gL-1 were the most effective in reducing bacteria in the fishpond after 24 h. Our study demonstrated that cinnamon extract could reduce microbial growth in water used to raise Siamese fighting fish.