I guess I should have been a little clearer...my point was:
Why feed freshwater/terrestrial foods to fish when there are similar inexpensive marine alternatives that are readily available, like dried nori, shrimp, scallops, etc.. at you local grocery store? I don't think that all of the foods cause health problems for our fish (but like I said they are more than likely not optimal...like the lettue thing for example), although I too have heard/read that feeding lionfish a diet consisting of only goldfish can lead to fatty change of the liver (look it up in a medical dictionary... "steatosis"). I don't know if this is fact, but like I said, if you can avoid it why not?
Also, the problem I guess is really feeding anything in excess...which would be the case if the fish's diet consisted almost solely of one or two items. This may not be your plan Lynn, so some of what I've said may not apply.
seven ephors, I see what you are trying to say...sort of. There is already significant research though that shows that there are specific beneficial effects of humans consuming fish due to the omega fatty acids that are almost exclusively found in fish oils. So people should be encouraging you to consume fish, and not "dissuading" you. OTOH, if someone had evidence for example that a certain goldfish protein was beneficial for lionfish, then I'd be all for feeding it to them.

Also, cow's milk isn't the optimal choice, especially for toddlers, children and young adults, but we haven't come up with a substitute that is both better AND cheaper. I think there was a decent article about it in Scientific American (or it could have been Time...I can't remember), but that was about a year ago already.