How big a tank you have?
You probably want to get a large enough fish so that the others can't eat it. I would go with another pictus, they like to be friendly and school, and their spines will prevent the oscar and JD from slurping it up.
You could try a channel cat as well. They get pretty big though at least 2' or more. I've a 4" in my 29 gallon with my goldfish, and he's a glutten. Will even come to the surface and slurp up the goldfish pellet, so I don't know exactly how much bottom feeding he's actually doing<G>.
~~Colesea