A VOLITAN in a 35gallon tank to small get a smaller species or get a bigger tank. You do know that this fish is going to be about a foot in length right?
Your next problem is the diet as stated before should consist of saltwater meaty foods it will love silversides, but mix it up krill, and any pretty much any other ocean food. How often do you feed it overfeeding is very common with these fish and will kill it eventually. I feed mine every three days and still watch out because it will continue to eat if I let it. They can go longer without food, Good luck! Im not an expert, but have learned from my own mistakes I was overfeeding mine when I got him and it developed hlle now I am feeding as stated abouve and treating his food with vitamins and he is almost healed up. Basically to put it blunt continue his past diet= sick/dead fish Fix the problem and you can still have a long lived healthy fish.
Good advice. IME & IMO: lions will live a long, healthy life if simple needs are met. A varied diet of the right foods, no over-feeding, added vitamins, enough room, and no triggers (etc) harassing them.