I mostly feed tinned sardines in spring water. I do have herrings in the freezer but haven't fed them yet. At the beginning of 'fish phase' I fed them tilapia minus the head.. Or was it mackerel, can't recall. The loved it, however I could not get over my anxiety over small pointy bones so I stick to the sardines.
People choose fish that are high in omega 3s and very low in mercury.