Not all dog foods are created equal and a dog eating poor food will not only look it, he may act it as well. Just as feeding a child a poor diet can effect his concentration and attitude, feeding our pups poor food can make a good dog seem hyperactive or listless. Research dog food brands and ask you vet for recommendations.
Some brands are more expensive, but in the long run are well worth the expenditure.
That is easier for the larger dogs, but to a 5 pound Chihuahua, a dog biscuit can be a big chunk of the daily calories. You can also try healthy veggie treats if your dog will eat them.