Matt,
Have you ever dewormed your fish? The fish in question, though its a bad picture quality, looks like it has some pretty extensive Hole in the Head going on, facial pits and erosions. These can be cause by alot of things, but nutritional deficiencies are one. Insufficient nutrition and water quality issues can also cause it, as well protozoans and worms.
My first suggestion since you don't want to medicate would be up the water changes to daily 50% to see if theres improvement, but I would suggest deworming as the next action to try.. I also might try getting a liquid vitamin and soaking their food in it or maybe expanding the menu to something more nutritious.
al