Do you know what specific type of pleco that is? One mistake that a lot of people make is assuming that their pleco has enough to eat simply because there is algae in the tank when in reality, not all plecos eat every type of algae. In fact, many of the most commonly found plecos (including those 'generic' types with non-specific labels at the store) tend to grow out of being mostly herbivores and become omnivores who are more than happy to eat flake and/or frozen foods that your other fish miss. The problem is, when your pleco cannot find enough food, or enough food to satisfy their nutritional needs anyways, it is very common for them to 'attack' tank mates...but they do so not out of spite, but in an attempt to feed from their highly nutritious slime coat. Typically, the best targets for plecos would be slow moving, non-schooling fish, including fish that tend to 'rest' at when the lights are off and when many plecos are more active. So, in conclusion, if you are not already doing so, you may want to supplement your pleco's diet. Of course, there are plecos who are more apt to behave like this than others so finding out what species you have is worthwhile. IMHO, these are fantastic fish and I have a few of them myself, however, their reputation as algae eaters is usually overstated.
I think there is also a chance that this fish may be suffering from a parasite as well. Have you noticed it glancing off anything in your tank, including the sand? Is the area raised and/or appear as though something may have come out of this spot?
Lastly, while I have used Melafix and Pimafix quite a few times, be careful with how much, how often, and even how you add it to your tank. I've read, heard, and even expereienced my own small disaster with using these products. Even though it is advertised as being 'natural', it can be overdosed. My suggestion is to use it sparingly and dillute it a bit in some tank water versus just dumping it in.
