Easiest way I've found (bettas get this, too) is to lightly drag a good, thick paper towel across the surface. And do it fast because you don't want much of that paper towel getting in your tank. Also, think about what you've been feeding him. If you give him greasy food, he'll make greasy poo. Bad for his liver, too. The oil/bubbles won't harm him as long as there is good surface aeration, but you'll want to make sure the nitrates stay below 20 as well for long-term health.
There wouldn't be any residue from having cleaned the tank, would there? Any soap-cleaned articles? Just have to ask because I'd be dumb not to.
Good that he's small! Keep an eye on your parameters and be ready to upgrade when he hits about 8".