When someone is underfed, they don't get enough to eat. It's worse to give your fish too much food than for her to be underfed, but it's still unkind to let her go hungry.
If you're underfed, you don't eat as much food as you need to stay healthy. When people are underfed for a long time, they become very thin and ill — you can also say they're malnourished or even starving. Animal rescue groups regularly take in underfed dogs and cats that need to be nursed back to health. This adjective comes from under, "below or less," and feed, from the Old English fedan, "nourish."