The Bubble Eye is a small variety of fancy goldfish with upward pointing eyes that are accompanied by two large fluid-filled sacs. It is a dorsal-less fish, and good specimens will have a clean back and eye bubbles well matched for colour and size. The bubbles are fragile and the fish should be kept separately from boisterous types and away from sharp tank decor, although the bubbles will regrow if punctured. The bubbles can disadvantage the fish as it is not a strong swimmer, with a seemingly low bobbing head at times, bubbles are infamous for being sucked into filters and siphons in an aquarium.
The Bubble Eye normally has an evenly curved back that lacks a fin. The pair of large pouches of skin attached under its eyes jiggle as it swims. Bubble Eyes have metallic scales and they are similar to the celestial eye goldfish. The eyes of the Bubble Eye goldfish are normal in the young fry, but will start to develop eye bladders three months after hatching. Like ranchu, the bubble eye goldfish lacks in dorsal fin and has a double tail. They normally grow up to 3 to 4 inches in length. If one of their “Bubbles” pop due to pressure or collision with a sharp object, there is a risk of infection where the inside of the sac has been exposed. source