Overall your Fern is quite healthy. If conditions are less than perfect, then some of the older leaves will discolor and gradually die back. To a limited extent that is inevitable. However, there are a few things you can do to limit the loss of older leaves.
It is too far off to the side of the window. It does want protection from the direct sun falling directly on it, but otherwise as much light as possible. Move it closer to the window or more in front of it.
Humidity is not an issue as long as you water it properly. As soon as the surface of the soil feels dry, water it thoroughly so some water trickles through the drain holes. Unless you water it multiple times per week, you need not be concerned about over watering it. Too much humidity is never a problem.
No need to fertilize your Fern more than 3 or 4 times per year and then only at half strength. Leaves that are unattractive can be cut off without harm to the plant.
