Abstract Background The present study was designed to quantify the contributions of water pH and hardness required for development and adult fitness indices of Culex quinquefasciatus (Cx. quinquefasciatus) mosquitoes. To this end, seven water pH conditions (pH 4.0–10.0) and five hardness levels (0, 30, 90, 150 and 210 mg/L CaCO3) were simulated following standard protocols. Day-old larvae of Culex quinquefasciatus were reared in these simulated water-media conditions till emergence. Entomological indices for immature developmental success and adult biological fitness parameters were measured. Result The results revealed significant effects of pH and water levels on the entomological parameters measured for the species. pH values of 4 and 10 and high hardness values ≥ 150 mg/L CaCO3 reduced immature developmental successes and adult biological fitness indices. The optimum range of values for the development of the species is respectively pH 5–8 and 0–90 mg/L CaCO3. Conclusion The present study reveals the significant negative influence of extreme pH and hardness levels on mosquito development and fitness indices; thus, it may be providing baseline information for developing sustainable robust vector control strategies for disease reduction through habitat manipulation.