Viscosity (thickness) and concentration are two different issues. Concentration has to do with the amount of surfactant in the shampoo and HydroSurge shampoos are more concentrated than most. The viscosity of the shampoo is an indicator of how much gum or salt is added to the shampoo. The low viscosity of HydroSurge shampoos is what allows them to work with our BathPro(TM) equipment, but if you are hand bathing using our shampoo, you will also find that this helps make the shampoos easier to rinse out.