Basically, you need clean water, swimming space, stress free environtment (IMO 8" is still alittle small to house a silver with say cichlids), if its eating feeders get it off them and onto a hikari cichlid gold or bio gold or aro food sticks, vary the diet as it grows to include freeze dried krill, fish cuts or even market shrimp (remove the shells though because they can gets tuck in the digestive tract and kill them, not worth the little extra red IMO, kinda like losing an aro because of vanity).
When I say clean water I mean waterchanges every week and canister filters or a wet dry. Swimming space, I would say a 75-125 would be good until they get to 18 inches then you will definitely need a much bigger tank, one with 30" width, or an indoor pond of some sort.