Another note is that they tend to be picky eaters, and in a small tank you will most likely pollute your 9 gallons quickly keeping a Cardinal feed.
I have one right now, it is one of three and the only one that survived. Why? The one that survived was the only one that would eat, and its picky still. I have quite a few different types of frozen and dry foods and it will only eat two of the items I currently have.