A photo might help.
If you do use essential oils, remember to dilute them with a carrier like olive oil, because some can burn and others are so concentrated they can be toxic if overdosed. Sorry if you already knew that, some don't.
My only guess at the moment regarding a potential cause is that it could be right-side heart failure. Right-side heart failure causes oedema of the legs, left-side heart failure causes fluid in the lungs. At least, in humans and animals with similar hearts, but chicken hearts are different from ours so I'm not 100% positive about that. However heart failure in many species often causes fluid buildup in limbs including in chickens. If it is heart failure chances are it's genetic and the animal won't last long.
But I don't know, there are many things which could cause fluid buildup in the legs. Is the chick herniated? What pose does it adopt? Is there any specific manner to its movement if/when it moves?