Prawn is not a natural food, i´ve never understood why people would use it except for plain commodity, which is a rather poor excuse. Fresh water shrimp, now that´s another thing entirely.
Personally i think the second most natural and ideal food for axolotls, or any aquatic caudate for that matter, are crustaceans. In a variety of forms, yes, but crustaceans are a big part of an aquatic caudate´s diet, hard evidence of that can be found in the bellies and coloration of many...many species.
I think we should all make an effort in introducing more crustaceans in our caudate´s diet, i know i´m trying with my limited possibilities (if i had a garden, oh how many things i would culture xDD).