Groundwater radon concentration of 83 Bq L−1 generated indoor radon levels exceeding 3,300 Bq m−3 at a commercial fish hatchery. Passive and active mitigation strategies to reduce the waterborne radon levels included a packed column, a waterfall through perforated grates, surface aeration, and bottom bubblers. Waterborne concentrations were reduced up to 83% using a combination of mitigation procedures, but a comparable reduction in indoor radon concentrations was not observed. A diurnal cycle showed that indoor radon levels peaked in early afternoon, probably as a result of warmer air being dissolved in the water during mitigation. Reduction of indoor radon levels below 148 Bq m−3 was seldom achievable with both water mitigation and direct air ventilation at 23 room air changes hourly.