This study investigates the role of water quality parameters as possible risk or protection factors in Aeromonas spp-affected rainbow trout hatcheries in northeastern Spain. The results revealed an association between oxygen concentration, ammonia concentration and total dissolved solids (TDS) with the prevalence of Aeromonas spp cases on the affected fish farms. The oxygen and ammonia concentrations acted as risk factors when their values were lower than 7.06 mg/L or higher than 0.05 mg/L, respectively, generating odds ratio (OR) values of 2.0 and 2.3. TDS levels, however, acted as a protective factor (OR = 0.39), from an epidemiological point of view but not statistically, when the values were outside the normal range, 199-261 mg/L. On a multivariate level, fish farms exposed to all the associated factors at the same time, had a risk of being affected by Aeromonas spp that was 1.74 times higher than farms with normal quality parameters; the risk increased when TDS had a normal value and it decreased when oxygen and/or ammonia concentrations had normal values.