Molecular variation contributes to the evolution of adaptive phenotypes, though it is often difficult to understand precisely how. The adaptively significant electric organ discharge behavior of weakly electric fish is the direct result of biophysical membrane properties set by ion channels. Here, we describe a voltage-gated potassium-channel gene in African electric fishes that is under positive selection and highly expressed in the electric organ. The channel produced by this gene shortens electric organ action potentials by activating quickly and at hyperpolarized membrane potentials. The source of these properties is a derived patch of negatively charged amino acids in an extracellular loop near the voltage sensor. We demonstrate that this negative patch acts by contributing to the global surface charge rather than by local interactions with specific amino acids in the channel's extracellular face. We suggest a more widespread role for this loop in the evolutionary tuning of voltage-dependent channels. In electric fish with ultra-brief discharges, Swapna et al. find a potassium channel that shortens action potentials by activating fast and close to resting potential. These properties derive from a patch of negative amino acids near the voltage sensor. Similarly placed charged amino acids may tune voltage sensitivity of channels more generally.