Aerogel and water Čherenkov detectors were employed to tag kaons for a Λ hypernuclear spectroscopic experiment which used the (e,e′K+) reaction in experimental Hall C at Jefferson Lab (JLab E05-115). Fringe fields from the kaon spectrometer magnet yielded ~5 gauss at the photomultiplier tubes for these detectors. These fields, which could not be easily passively shielded, would result in a lowered kaon detection efficiency if not mitigated. A bucking coil was placed on each photomultiplier tube to actively cancel this magnetic field, thus recovering kaon detection efficiency. •Bucking coil was developed to cancel a magnetic field for a photomultiplier tube.•A gain reduction of the PMT due to magnetic fields was carefully studied.•Bucking coil conditions were optimized to recover the gain of the PMT.•Bucking coils implemented in a hypernuclear experiment worked well.