I agree with Bryan. Overfeeding, especially with feeder goldfish, causes the demise of many lionfish. Goldfish are coldwater species, and thus have a lot of fat.
It may take some doing, but try to "train" your lionfish to other foods. This can sometimes take weeks, depending on the lion. You can try any of the frozen foods, like prawn, silversides, or buy fresh stuff. I initially got mine trained OFF of goldfish by dragging pieces of imitation crabmeat along the bottom of the tank attached to a thread. It might take a few days of doing this..don't lost patience. Lionfish respond to color, so use the orange portions of the crabment, thread a needle, pull it thru the piece of food (don't tie a knot, if the lion grabs it, you need to pull the thread out gently)
Another easy thing to train them to is earthworms. My lionfish love earthworms. I keep a container of live ones in my fridge. When they get hungry, I just grab a worm, cut it up with scissors, and toss them into the tank. Lions LOVE these. They will BEG you for more, don't give in.
You can also train them to eat chunks of Formula One. Use the thread if necessary. Make sure your lionfish is HUNGRY. That means not feeding him for a few days. And once trained, feed him a variety of foods, and not more than three times a week. Twice is fine.
HTH