We should be able to put the Fillet Knife in the Tacklebox to keep all of the fishing gear together. I'm on Neoforge 1.21.1

The Tackle Box is only intended to be able to store fishing rod and fishing rod equipment, which the Fillet Knives does not all under.

It is possible to add whatever you like to the aquaculture:tackle_box item tag, which will allow the Tackle Box to store whatever you define in the tag.