Certain species like arowana and oscar prefer meaty requirements in their diet while others are less picky when it comes to food and will generally accept anything that you feed to them. Providing wrong diet can be disastrous in the long run because the fish will end up malnourished and that could even cause them to be susceptible to aquarium fish disease.
For me, the most important factor to get a healthy fish is to provide them with a good mix between dried fish food and live foods. Carnivorous fish would require larger proportion of live foods while omnivorous cichlids and other tropical species need balanced mix between both, so it’s up to you to come up with the feeding schedule accordingly.
Today there are even different aquarium fish supplement specially formulated for certain requirements. These special foods are made with high protein used as growth enhancers especially for fish fry, extra vitamins and minerals to bring out special bright fish coloration, oils formula for growth of goldfish wen (headgrowth) and there are even medicated well-balanced nutrition for sick fish.