Fishes anatomy and our anatomy doesn't differ that greatly. When you give them a guppy, think of it as if it's a steak for you. You could, but probably wouldn't want to live on just steak, infact it will be healthier for you to add vegetables and other stuff into your diet. Same thing goes for fish. Go ahead and give them a hunk of meat evey once in a while, but don't make it their only food. Infact I don't think you want to make anything their ONLY food. What's everyone elses opinion on supplying a range of foods for your fish?