Oils do not dissolve in the water. Use of oily products in the aquarium will result in a scum that forms on the surface and can cause problems with gas exchange, especially in tanks with little surface agitation. Quaility fish food provides for the fats and oils needed by the fish. Supplementing it through food would, IMM, be hazardous in terms of selecting an appropriate product, much less dosage.
Keep in mind that fish biology is very different from mammalian--the effects would likely be different from those you are used to. Garlic is occassionally used, though there is not much scientific support for it. Clove oil is used as anesthesia--or for euthanizing a sick fish.