Repot your fern after nine months to a year of growth. You should allow the plant to dry out just a bit and pull it out of the pot to examine the roots.
If the roots are to the edge of the pot and are filling out to the base and circling around, then it is time to move up to a larger size pot.
Usually increase the size of the container by 2 to 4 inches in diameter. You can also divide the plant in half, using a steril and clean, sharp knife.
Depending on the variety of fern, some may be pulled apart and divided into seperate ferns.
Gently loosen the roots before you repot your fern for better soil contact.
Restrict the plant food for about a week after you have disturbed the fern's root system to help soften any transplant shock that may occur.
Ferns LOVE plant food, most are heavy feeders, so I recommend incorporating a slow-release plant food in the soil and a bit on top at soil level.
So long as your plant survived transplanting, water (they prefer room temperature drinks) with a diluted strength all-purpose liquid plant food formulated for houseplants