We can be brief about this subject. Galanthuses do not like fertilizer! Most likely this is due to the fungi living in unison with the plant on the surface of and inside its roots: the Mycorrhiza.
When planting the bulbs, it is important that sufficient humus or organic material is worked through the soil. That way, the bulbs can grow for years. In early autumn, one could add a small layer of humus on top of the bulbs. Leaves fallen from trees are suitable as well. These leaves decay into humus and eventually into nutrients for the bulbs.
The only thing you may want to add immediately after flowering is a little lime.
For seedligs and twin-scales it is advisable to add a little liquid fertilizer twice during growing season. But again, if you have added sufficient leaf mould to the soil in which is sown, or in which the twin-scales are planted, then no fertilizer is necessary.