i think this has been a misunderstanding, based off people using freshwater FISH and food for saltwater fish. i've seen a lot of debate back and forth, primarily in regards to people feeding goldfish to lionfish. the primary issue being an incorrect HUFA profile and possibility of fatty liver.
http://www.saltcorner.com/Articles/Showarticle.php?articleID=70
the other facet of this is feeding certain terrestrial plants like garlic or romaine lettuce, which is also apparently undesirable for long term health.
i think these two points have been conflated quite a bit, and many people have extended that to all foods.
i don't feel that using terrestrial versus freshwater versus marine foods as a blanket guideline is the correct way to go about it though.
for example, white worms are a mostly terrestrial worm, but provide an excellent nutritional profile:
https://www.google.com/url?sa=t&rct...x8jdWUYxLAsnBJeeQ&sig2=5eV5LEnBmIXtRLUtfuljiw
this puts them pretty high up on the list as far as nutritional content:
http://www.caudata.org/cc/articles/foods2.shtml
they're actually roughly on par with anchovies in that regard.
so i think making the broad statement "don't feed freshwater/terrestrial animals to marine animals" is an incorrect statement to make. there is considerably more to take in to consideration aside from where the food happens to reside.
