Krill has been linked to lockjaw in predator species like lionfish. It’s best to get him off of it ASAP. I don’t even feed it occasionally because some fish will look at it like candy and eat nothing else. A lot of lions can be hard to switch to frozen food. The only way I could get my zebra will eat is if it is still moving. If it lands on the bottom, he won’t touch it until I blast water at it with a little baster to get it to move.
In case of physical ailments let me ask you these. How long have you had it and has it ever eaten for you? How long, if you know, has it been since it last ate? Is there any damage to the fish or cloudy eyes? What are it’s tank mates if any? Any anemones in the tank? These are perching fish and can get stung or even consumed by an anemone. Look for dark sting marks on the fish if you have any.
Also remember that lfs are there to make money and will not always be truthful. They may have flat out lied to you that the fish was eating or maybe the person saw it eat once and assumed it was eating like normal. Always ask to see a fish eat, especially if it can be a picky eater. My lfs which I trust hired a new person and he told me that their new leopard wrasses were weaned from frozen and eating rods food. I waited until one of the older guys were in there and they in fact informed me that the leopards were not eating frozen.
Most likely cause of this hunger strike is that the lionfish doesn’t eat prepared foods. They are ambush predators and not scavengers so you need to find something it likes and ween it to frozen. As for food try some different stuff. Mine will eat mysis, but they are small and take a while to feed him. Jumbo mysis by Hikari are bigger. Go to the grocery store seafood counter and buy a fresh shrimp, scallop and maybe a clam. Cut into bite size bits and see if it will hit any of it. My zebra likes shrimp but spits out scallops. You can also spear a piece of the food on a bamboo skewer or feeding rod and “dance” it in front of the fish. If none of this works your best bet is gut loaded brine shrimp or ghost shrimp. Ghost shrimp are almost irresistible but will get expensive if you can’t get it to eat frozen. If you have a place that sells live brine shrimp get a little and mix some frozen mysis with it. It may eat the mysis as it’s is chasing the brine and decide it is a valid food source.
Stay away from freshwater feeder fish if at all possible. They don’t have the proper nutrition long term for a saltwater species. If it is between them or starving though, gut load them first. Basically just feed the live food some quality marine flake or pellet food right before feeding it to the lion.
