Your 'Fantail' is actually a strain known as an 'Oranda', these are selectively bred to produce hood-like growths on their heads, it is perfectly normal and even if the fish did lose its eyesight, it will still be able to find food using the lateral line system.

The only thing I would worry about is the tank size, those goldfish can and will eventually grow to 15cm/6" in length, so you would be looking at a 90 x 38 x 38cm tank.