Im not saying fillers and soy are good.. quality protein is key to good growth and development. However as you said Discus are thought to be omnivores. So should be able to utilize a variety of foods containing plant matter provided their protein needs are met. I'm just taking the opposing point to make this discussion more interesting
Looking at the label is fine as is setting the bar high nutritionally but theres also another factor..palatability. it can be the most nutritionally balanced and healthy food but if the animal won't eat it.. forget about it.
NLS is such a food. For some reason its not well accepted by discus. Its been brought up over the years here on the forum.