The influence of the nature of three representative monovalent co-ions on the gelation kinetics of Ludox suspensions has been investigated. At a given Ludox volume fraction and for the same concentration of potassium salt, the gelation time is longer as the studied anion presents a more pronounced kosmotrope character. As the screening of the silica surface charge is similar since the same cationic counterion is taken, these results highlight the unexpected role played by hydration effects imparted by the co-ions when particles are pushed together as gelation proceeds. This reveals that jamming transitions of nanoparticle fluids may be finely tuned by changing the co-ion nature in spite of the fact that the cationic counterion is the same.