Just like in cat food and dog food brands--quality can vary a -lot- and the cheaper ones will use a lot of fillers like wheat and ash--I see that you have a dog, so I'll use a dog analogy, because it's kind of like how really bad dog food will have crushed corn as the first ingredient and also include things like "mill run." Check the ingredients and apply the same principles you would when picking a dog food--you want a whole meat to be the first ingredient, such as whole salmon, halibut, shrimp, krill, squid, etc--if it's followed by a meat "meal" even better. The further down the list you find grains like wheat or soy the better.
Brands like Hikari are now like the Pedigree or Beneful of betta foods, lol. In other words, total crap. Apply the same ideas as you would to buying food for your other pets and you should have no problems. For instance, here are the ingredients in OmegaOne Pellets:
Whole Salmon, Halibut, Shrimp, Wheat Flour, Wheat Gluten, Fresh Kelp, Astaxanthin, Lecithin, L-Ascorbyl-2-Phosphate (Source of Vitamin C), Natural and Artificial Colors, Vitamin A Acetate, Vitamin D3 Supplement, Vitamin E Supplement, Vitamin B12 Supplement, Riboflavin, Niacin, Pantothenic Acid, Folic Acid, Biotin, Inositol, Tocopherol (Preservative), Ethoxyquin (Preservative).
This is what I consider to be a decent ingredients list--a variety of whole meats are offered, along with color enhancers like Astaxanthin (enhances red and orange coloration), and a list of vitamins and minerals.