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A mosquito’s life cycle includes an aquatic phase. Water quality is therefore an important determinant of whether or not the female mosquitoes will lay their eggs and the resulting immature stages will survive and successfully complete their development to the adult stage. In response to variations in laboratory rearing outputs, there is a need to investigate the effect of tap water (TW) (in relation to water hardness and electrical conductivity) on mosquito development, productivity and resulting adult quality. In this study, we compared the respective responses of <i>Aedes aegypti</i> and <i>Ae. albopictus</i> to different water hardness/electrical conductivity. First-instar larvae were reared in either 100% water purified through reverse osmosis (ROW) (low water hardness/electrical conductivity), 100% TW (high water hardness/electrical conductivity) or a 80:20, 50:50, 20:80 mix of ROW and TW. The immature development time, pupation rate, adult emergence, body size, and longevity were determined. Overall, TW (with higher hardness and electrical conductivity) was associated with increased time to pupation, decreased pupal production, female body size in both species and longevity in <i>Ae. albopictus</i> only. However, <i>Ae. albopictus</i> was more sensitive to high water hardness/EC than <i>Ae. aegypti</i>. Moreover, in all water hardness/electrical conductivity levels tested, <i>Ae. aegypti</i> developed faster than <i>Ae. albopictus</i>. Conversely, <i>Ae. albopictus</i> adults survived longer than <i>Ae. aegypti</i>. These results imply that water with hardness of more than 140 mg/l CaCO₃ or electrical conductivity more than 368 µS/cm cannot be recommended for the optimal rearing of <i>Aedes</i> mosquitoes and highlight the need to consider the level of water hardness/electrical conductivity when rearing <i>Aedes</i> mosquitoes for release purposes.