When tamed slabfish is equipped with a chest (backpack), it will duplicate and drop the items in the inventory if you middle click (scroll wheel) the inventory. Some items (i.e., from Mekanism) will 'break' this and delete the chest and drop the items, but otherwise it could be a virtually infinite supply. Playing on Enigmatica 6 Expert version 1.1.2, (environmental-1.16.5-1.1.1).