In a way, that is a good problem. If your child is active and healthy, they are probably getting enough good nutrition. One would need to know what the rest of the family looks like and the child's activity level. I think the one thing I have been able to link with weight gain in kids is the amount of cow's milk they consume. Your pediatrician will have growth charts available.
Fish is a good source of animal protein.
We are finding some of the fats in fish are high in essential fatty acids, which are crucial for a kid's development. It is important because it is a good source of protein and good nutrition. Some kids don't like exercise for exercise sake. They want to do fun activities and not drill-like activities. Some kids like well-refined parameters and some kids don't and might rebel from it.