Is an animal’s Nutrition rating lower than you’d like? You may want to bump up your food quality. The more you research a species (with Vets), the better food you’ll be able to produce, but there’s more to it – you have to change the meal plan manually.
Click on a habitat, switch to the “Animals” tab, and change the food quality from Grade 1 to Grade 2 or Grade 3. If you don’t have these options yet, you need to do more research. The higher the grade, the pricier the food, so don’t go bankrupting yourself.