Effects on mosquito breeding, acidity, and fish presence were assessed after installation of a small tidal leakage port into an acidic drain's tidal exclusion flap gate. Before gate modification the drain water had pH values as low as 2.7. These water conditions were toxic to fish and continually held mosquito larvae. At 4 wk after gate modification, water pH had risen to 6.0, at least 3 fish species inhabited the drain, and mosquito larval numbers had been reduced by 99.98%.