Purina is a poor quality food, and it may explain why you are having health problems with your dog.
Ear infections, red runny eyes, are often caused by a reaction to eating a food heavy in corn and grains. Dogs need meat to thrive.
Look for a food with no corn, wheat or soy and that does not contain by-products.
In Canada two of the best foods are Orijen and Go Natural. Some great American made foods that are sold in Canada are Innova, Canidae, Wellness, Natural Balance, Eagle Pack Holistic, California Natural,etc.
Honestly, almost any food will be an improvement over Purina, and you will have to feed far less of a good quality food.
Gina