Ground-supported circular reinforced concrete (RC) water tanks exhibit seismic responses governed by liquid–structure interaction, tank geometry, and site conditions. This study evaluates six fixed-base tank configurations with a constant internal diameter of 15 m and liquid-depth-to-diameter (h/D) ratios of 0.35–0.60 using a spreadsheet-based equivalent impulsive–convective spring–mass model based on IS 1893 (Part 2):2014 and the IITK–GSDMA Guidelines. Mass participation, natural periods, global seismic forces and moments, wall-base seismic pressure, and sloshing amplitude are evaluated for hard, medium, and soft soil spectra. As h/D increases, the impulsive mass fraction increases from 0.398 to 0.620, while the convective fraction decreases from 0.564 to 0.374. For hard soil, base shear increases from 1.815 to 4.218 MN, the global wall-bottom moment from 4.130 to 15.606 MN·m, overturning moment from 9.873 to 26.412 MN·m, and combined seismic pressure at the wall base from 20.557 to 32.055 kPa. Logarithmic regression using all six configurations identifies the global wall-bottom moment as the most sensitive response, with an exponent of 2.468, followed by overturning moment (1.826), base shear (1.566), seismic pressure (0.825), and sloshing amplitude (0.118). Soft soil produces greater convective demand, while sloshing increases by only 6.62% across the investigated range. Nevertheless, all calculated sloshing amplitudes exceed the assumed 0.50 m freeboard. The findings highlight differing sensitivities of global structural and sloshing responses and apply to configurations in which increasing h/D simultaneously increases liquid depth, storage volume, and mass.