When salts are added in small quantities to an aqueous solution, its surface tension increases due to the dielectric discontinuity at the air/water surface. This idea was implemented in the pioneering work of Onsager and Samaras, who found a universal limiting law for the dependence of the surface tension on the salt concentration. However, the result implies an increase in the surface tension which turned out to be violated in many physical realizations. We employ field-theoretical methods, and extend the Onsager-Samaras theory in a self-consistent way by considering ion specific short-range interactions with the surface. Expanding the Helmholtz free energy to first-order in a loop expansion, we calculate the surface tension of electrolyte solutions at the air/water and oil/water interfaces. Our theory fits well a wide range of salt concentrations for several different salts at the air/water interface, and dodecane/water interface, using two fit parameters: one related to the ionic size and the other one to the ion-surface interaction. We also obtain analytically the surface-tension dependence on the ionic strength, and its variation with ionic size and the ion-surface interaction, reproducing exactly the reverse Hofmeister series for monovalent anions. From our analytical result it was found that for uncharged surfaces, and as long as the weak-fluctuation regime is valid, the fluctuations always dominate over the mean-field.