Feeding fresh water feeders to a lion fish will result in a short lived lion fish.
Lion fish will eat anything but you have to work with them for a while. It's just conditioning (teaching) them. I made a post about this yesterday. Lion fish can be and are often stubborn fish, espcially big ones (a 6" dwarf is a big one) Start it out on something live that you can duplicate dead. I like to use live ghost shrimp, they normally go for that. Once the lion is to the point where it is at the surface waiting for you to drop the live ghost shrimp in, you can start tricking them with dead ghost shrimp or frozen (thawed out) krille. Lions will eventially get to the point where they will eat anything. You should not feed lions meat only, espcially don't get them hooked on shrimp & krille. During this "learning" the lion will need to be in an aquarium eigher by it self or in a tank with little compition. I feed saltwater meats like squid, shrimp & fish but I also feed the salt water fish foods that have algae and vegtable matter in it. I like Ocean Nutrition's formula foods like one & two. If you feed a meat diet only, it will not live long. If you feed it gold fish, guppies, roseys, etc, it's life will be even shorter.
It is (was) very irresponcipal of an aquarium shop to suggest to feed live "feeders" to your lion. It doesn't sound as if these people are very knowledgable....Joel