The first thing that can be the cause of yellow leaves is a lack of proper nutrition. However, when this happens, most of your fern leaves would turn yellow, not just a few. They may also be getting suffocated and need a proper room to thrive. This can happen if you have planted your ferns in an extremely small pot. Ferns do not require too much sunlight, but a complete lack of sunlight could be another reason for the leaves' yellowing.