First off, you can't keep a common pleco healthy in a 30 gallon. They grow to 2 feet in length... he is stunted now and having issues. Please don't use salt, that is bad advice, it's not going to help his condition. You need to test the water, get that in order, but then this is a condition that possibly can't be treated, and you can't keep him in the 30 gallon either. You need like a veterinarian specialized in fish, have him picked up by the Ohio Fish Rescue or possibly have him euthanized.