Dog kibble fed once in a blue moon may not be bad. But, regular feedings can cause problems. Most kibbled (dry-processed) dog food contains meat meal and bone meal along with meat fats. Fish and bio-filters have a hard time processing mammal fats. Kibble also usually contains blood meal and offal (organ meat and waste trimmings) that can cause sudden nitrate swings in water chemistry. The average ash content of kibble is 8%. That's material left over after full digestion (unmetabolized material) that will only clog a filter with no chance of breaking down.