Treating my horse's skin condition

At my horse's vet visit on Friday, the vet gave me instructions for treating a possible skin ailment around Panama's ears. Although it sounds like the problem may actually be due to Panama's ear mites (the vet gave me a treatment for ear mites too), the vet is concerned that it could also be a fungus.

Basically, the vet told me to mix a cup of bleach in a gallon of water, then clean the affected area with the mixture once a day. This is the same mixture we used to clean tables when I used to work in childcare, so I can also tell you that it loses its disinfectant abilities after 24 hours — meaning you have to make it fresh every day. (In other words, don't make a whole gallon at a time, unless you have a very large area to wash.)