Many years ago, in a fish forum far, far away, a question in the form of a negative comment was raised about fish foods that contain fishmeal. Being unsure about fishmeal, I did some pretty thorough research....
Fishmeal is often cannery refuse that's ground, dried and loaded with preservatives to sit in a warehouse (sometimes for months on end) until ordered by a fish food manufacturer. Because it is dried and lacking fresh protein, copious amounts of starch (usually from grains - such as wheat, rice, oat, gluten) are required as binder/filler. But fish can't use/process grains, so it passes through as excess waste.
Higher quality fish foods use fresh fish (protein) which requires no (or far less) grain as binder.
So check the label and see what's listed first in the order of ingredients!
Note: when I switched those many years ago to high quality foods I noticed that the waste my fish produced was SIGNIFICANTLY less.....'believe it or not'.
Tip: to save a few dollars on the more expensive, high quality foods, buy the large size and store in the freezer, keeping only a small container for regular feeding.