Koi fish (Cyprinus carpio) is one of the very potential types of ornamental fish in East Java and has a high economic value. Aeromonas salmonicida is a bacterium that includes Quarantine Fish Disease and has spread on the island of Java. Various antibiotics are available on the market for treatment of these bacterial infections, however, only commonly used certain antibiotics. Continuous use of an antibiotic can spur bacterial resistance. This study was conducted to test the effectiveness of ciprofloxacin in the treatment of A. salmonicida infection. Ciprofloxacin was chosen because it has the highest inhibitory value of A. salmonicida growth of 10 ppm with 11 ppm MBC value. At these doses up to 10 times the value of MIC, ciprofloxacin did not show a toxicity reaction in Koi fish. The researche design used Complate Randomize Design with different ciprofloxacin doses, namely treatment A (11 ppm), B (16.5 ppm), C (22 ppm), D (27.5 ppm) and E (33 ppm) repeated 3 times. The results of data analysis with ANOVA showed significant differences between treatments (p <0.05) Analysis continued with least Significant Difference and showed that the treatment of 11 ppm was significantly different from the treatment of 22 ppm, 27.5 ppm and 33 ppm. The treatment of 16.5 ppm was significantly different from the treatment of 22 ppm, 27.5 ppm and 33 ppm, while the treatment of 22 ppm, 27.5 ppm and 33 ppm was not significantly different, the dose of ciprofloxacin was effective and efficient or best for the treatment of A. salmonicida in Koi fish is 22 ppm.