Most farm feeds of this quality will have a 40-60% inclusion rate of lower cost grains/starch. Not as "fillers", but as a cheap source of energy.
Carbohydrates are not a problem when used at lower inclusion rates, and most species of fish have the enzymes to break them down & utilize them as an energy source, but not at excessive levels such as what's found in these types of farm feeds.
Kibbles & Bits is cheap too, but I wouldn't feed that to my dogs, anymore than I would feed Purina to my fish.
Carbohydrates are not a problem when used at lower inclusion rates, and most species of fish have the enzymes to break them down & utilize them as an energy source, but not at excessive levels such as what's found in these types of farm feeds.
Kibbles & Bits is cheap too, but I wouldn't feed that to my dogs, anymore than I would feed Purina to my fish.