http://www.drsfostersmith.com/Produc...&N=2004+113017 - You might find this locally or have to order it, but I'd treat the tank with this. It sounds like the disease has spread to at least 1 other fish and since symptoms don't always show right away, the best thing would be to treat them all and knock that disease out of the tank.
What are the current ammonia, nitrite and nitrate readings? How many fish are in there and how big is the tank? Are any other fish showing any changes in appetite or appearance? How often and how much % do you do partial water changes?
I take it, this is in the 20g. It's overstocked with the common pleco and with the amount of waste that plecos produce, water chemistry would be a problem and possibly the cause of the popeye & HIH disease.