Fish do not have the complex nervous system we do. Sometines when a fish has ich, or a fungus, you will see them rubbing against decor and what not. This looks like they are scratching, and it may be that they are. But I don't think it is an "itch" as we know it. If you have ever been fishing and had a duck accidently take your bait, they do not fight like a fish. The come as quick as possible. This is because the duck has many more nerve endings than a fish, who would fight you all the way to shore.