Personally I believe it's predominantly due to the fat build up behind the eyes, so diet and lack of exercise could be the root cause. Obviously your very large Silvers need tanks that rival small ponds in size, plenty of turning space and ideally a wave maker for maximum exertion. I've been very strict with my XB's diet from the very beginning. His staple food is Blaptica Dubia Cockroaches and Super Worms. These Roaches are higher in protein than crickets. The worms are slightly fatty so I scrape out most of the insides of about 95% of them before feeding and stuff/inject with whatever I choose. Hikari sticks, slither of garlic (good for insides), Aro vitamins, Spirulina and fish oil once a week. He doesn't eat fish so I make up for the lack of this essential fatty acid with this. He also gets the occasional crickets, bull frog, earthworm and Gecko. All being low fat, especially in moderation. Im glad I've 'tricked' mine into eating SW no matter what they're filled with as I can really control his intake on all levels. Just a few ideas for you. Good luck!