I'm not sure about the fat but first of all,all the frozen smelt that i have bought has had this weird chemical smell and years ago I gave my ten inch armatus a few pieces and a day or two later it went belly up.More recently,about two months ago I fed some to one of my perch and it has been acting odd ever since.I thought it was going to die but it has been holding on so far.I had forgotten about the armatus and didn't recall until after I noticed the perch acting weird.
I can't pinpoint it but there is something funky going on with frozen smelt.