I have a ficus benjamina which I acquired from a friend who moved away several years ago, and it's my only house plant. It's rather unhappy at the moment, but not in the usual way - it's not dropping leaves. It just looks sad. I've only ever given it water, so I was wondering if it needs something more. Some kind of plant food? A change of soil? Does anyone know what would make it perk up a bit? I know nothing about plants so would be grateful for any advice.