I'm repeating myself here, but;
Variety and underfeeding is the key to a healthy Lionfish diet.
Krill is okay as part of the diet, but you must rotate with fresh chopped squid, shrimp, silversides, krill, (frozen or freeze dried). IME, a live food treat now and then (one a month or less) of ghost shrimp or crickets will be appreciated. None of these should be allowed to become a single food item, there are some authors who site complications with lionfishes related to a diet of krill only--including blindness, and for those of you who pounced on the fresh water guppies, I was very clear about using them as a last resort, to induce feeding NOT a regular food item. :mad2:
That cave should work out well for you just make sure it's stable, best wishes for your Lionfish my friend.