Ah...the krill saga. It has been linked to lockjaw in many predatory species, and the entire story is a long one. However, being a nutrition geek, my wife did lots of research as to "why"...
Krill has a shell (exoskeleton), and this shell contains excessively high amounts of fluoride. In diets high in krill, this fluoride affects the growth and condition of the fish's mandibular structures, resulting in malformation and other problems, thus causing some fish's jaws to lock open upon feeding.
But the fun doesn't stop there...krill flesh also contains high amounts of the enzyme Thiaminase, which inhibits the fish's uptake of Thiamine (vitamin B1). This can lead to central nervous system disorders such as anorexia (cessation of feeding), seizures, clamped fins, and even an early death.
Some suggest shelling the krill prior to feeding it out, however, over time, fluoride and other things can move from the shell into the flesh of the krill, altho no study has mentioned how long this takes, but the longer the krill is dead, the more likely it is for this to occur.
FINALLY, krill contains absolutely ZERO vitamin C...like goose egg. Vitamin C is very essential to healthy fish.
Now add to all this the fact that krill contains a natural attractant, which is why many fish accept it as a first food, and once many fish are "addicted", they refuse to accept other foods.
SOME krill is OK as long as you offer a good, varied diet, however, we personally, don't use krill at all. A better (and cheaper) alternative are pieces of shell-on table shrimp. With a puffer, you also want to offer snails, small mussels, clams, or live fiddler crabs...all shell-on to help keep the fish's teeth worn down.
HTH a bit...
