Looking at the mouth, teeth, jaw structure, and even body shape of any fish will tell you a lot about required diet.
And it may change as the fish ages. Note the structure of a young O goramy's mouth, pointed, able to access insect larvae and crustaceans in the substrate, and between rocks, so more proteins may be required. when young..but....the shape changes as it grows, gets more upturned for accessing fallen fruit, leaves, surface dwelling aquatic plants, etc.
Most fish (even primarily vegetarians don't pass up easily accasable protein, but have longer digestive tracts that get the most out of vegetation, and may be irritated by too much protein.

Above an Xray of the primarily vegan Cincelichthys pearsei.

compare the dental structure of a carnivore above, with the vegetarian, Cincelichthys bocourti below

And compare the flat platelike bodies of those that tend toward vegetarian, to the streamlined carnivores.

The only veg I tended to avoid for my goramy's and other primarily plant eaters, was broccoli, because it made my fish room smell like a beer and sauer kraut fart fest.