first of all, to take any fish out of any river, lake, ocean and keep it in a confined space such as home aquarium is some what sadistic. why we keep fish in aquarium? isn't that for our personal enjoyment?
second, now you have to remove part of what's built into its DNA, isn't that wrong? we've been doing that to many other animals such as lions, tigers, etc. in the zoo. keep them well fed is one thing but you think they rather run free in the plains and forests hunting preys or enjoy being locked up in a cage and being feed like some soulless shells?
for those who totally against feeding live and that's fine, do whatever makes you happy. However, that doesn't mean those who decide to let the fishes follow their instinct are wrong and should be criticised, as long as the feeders are healthy. We do not jump on you for removing part of nature out of the predator's gene by feeding them dead meat. We respect what you do to your fishes and hopefully you would respect what we do as well, there's no right or wrong here.