I think variety is the answer. If you feed a variety of foods the fish ought to get all the vitamin requirements they need.
I have found, from personal experience, the main difference beteween the cheap and expensive foods was the amount of "bulk" if that's the word. I had some cheap pellets and some dearer ones. The fish did not like the cheap ones and refused to eat them. When I added one of each to water, the cheap ones swelled up twice the size of the dearer ones (due to a "bulking" agent?). This caused problems for the one fish who did eat the cheap ones - major balance problems as it had difficulty digesting the pellets as they bulked up more inside the fish and it couldn't swim right until it finally managed to get the pellet through its digestive system.
More to the point is to use fish foods quickly once the container has been opened as they lose their 'goodness' rapidly once exposed to air.