You're feeding your lion a food item marketed for our hobby and more specifically predatory fish. You're enriching these food items with a well known trusted enrichment that has been around forever and used by many. Your fish has been nice and fat.... why would you even think he would not be receiving all he needs, right! The occasional goldfish will be of no consequence, just pick a small one and not one of those with the massive heads.
Do you have access to a market seafood deli? All the saltwater fare will be great for your predators. Mix up fish flesh and inverts because they all have something great to offer and something not so great to avoid. Fish high in fats have a much shorter shelf life due to lipid rancidity or oxidization. Inverts tend to be high in thiaminase, Many things are low in vitamin C and the by the time it reaches their mouth, there really isn't any left. So supplement that when you can. I use Brightwell Vitamin C and add it at double of the recommended doses given on the side of the bottle. I've tested the ascorbic acid of this product and the levels were still satisfactory.
Cod, tuna, shark, salmon, clams, table shrimp, squid, haddock. If you need more specific help in that area, just let me know. Lobster is very snotty when raw, so avoid that.
But you have a hard road ahead of you to get this guy to improve. Definitely get some vitamin C in there if you can as a first step. Can you remove him from that tank? Moves are stressful, but he has to get away from the puffer. I had a lion get the snot beat out of him every other night and we never saw them fight. They can also be compatible one moment and not so friendly the next due to sexual maturity. All of a sudden space, food and woman are worth fighting over.
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