You want to read the dog food label on your dog’s food. Ingredients to avoid are: gluten/gluten meal, by-products, corn, wheat, soy, sugars (often labeled as whole ground sorghum), BHA, BHT, ethoxyquin and dyes. And remember some dogs do better with absolutely no grains in their diet at all.
Adding a probiotic to your dog's daily food is very good for the gut and is highly recommended for all dogs but should be used for anyone whose dog has any GI problems of any kind. It can often help with some behavior problems. (Ask a knowledgeable trainer about this.)
Try adding the following fruits and veggies to your dog’s diet: broccoli, cauliflower, green beans, carrots, peas, tomatoes, berries, watermelon, apple, cantaloupe, pineapple, bananas. You can also add yogurt, plain or flavored (stick to no or low sugar; plain is preferred). For nervous, shy, anxious, timid as well as aggressive dogs it is highly recommended that you add bananas, figs and yogurt to your dog’s diet to help produce more serotonin in the brain.
DO NOT feed your dog onions, apple seeds, grapes, or raisins as these can be toxic & deadly!
You can now purchase many quality foods in Petco stores, not Petsmart, but Petco. Petsmart hasn't gotten on the bandwagon for good nutrition just yet. I'm not sure why not but . . . Also try to stick to white meats/proteins: fish, chicken, turkey, duck . . . you get the idea. Stay away from red meats, dogs just tend to do better with white meat protein.
Good diets I like and recommend are:
Halo: Spot's Stew Chicken Recipe
(found in Petco stores)