It definitely makes a difference. After having fish for 11 years and working in pet shops for 6, I consistantly see that the more aggressive individuals are fed live. This doesn't mean that one fed pellets will refuse live, they are still predators, but no live food ever does almost always lead to a less aggressive individual. Also, this does not mean that the fish is not aggressive, just less aggressive that it would be if you fed it live. Most fish will take to pellets. Hikari's Food Sticks are designed specifically to get fish off live food. Usually it just takes patience and not feeding for about a week or more to get the fish hungry enough to take pellets. There are few species that will effectively never take pellets. I have had alligator and hujeta gars switch over without much problem. It make sthem more aggressive, is nto even close to nutritionally complete, and greatly increases the risk of (or arguably guarantees the eventual problem with) parasites and diseases. So in my opinion and experience the only fish that should get live (outside of breeding situations) are those that continually refuse to switch over to pellets.