[en] 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. -- Highlights: •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