Common stillwells attenuate slosh effects in two ways: a decrease in pressure variations with depth and the damping caused by orifices. Often the slosh frequency is near the resonance frequency of the stillwell. Addition of an “inertia tube” can lower the stillwell natural frequency sufficiently to detune the system without seriously increasing the head loss due to propellant outflow. Even in a very short stillwell, where the depth effect is negligible, the “inertia tube” can greatly attenuate sloshing. This paper treats the stillwell as a vibrating system and presents equations for simple stillwells, orifice stillwells, and inertia-tube stillwells.