What is someones filler is another's nutrious ingredient. Some thinks to keep in mind, Most of what you read about dog food is nothing more than marketing B.S. ie corn is a filler. All kibble to hold its shape require a source of starch, the absolute minimium is 19% and at that it is still not really economical practical. So every dog food "kibble" uses some sort as starch "carbohydrate" to act as a binder and add calories and some nutritional content. Those dogs foods that rail against corn, rice, wheat and other grains, simply use other starch sources like like potato or tapioca for which it could easily be said are actual less nutrious than the grains they replace.
What Are The Most Healthy Grains, Legumes And Starches?