My comet goldfish, had his tail bitten by a Piranha (don't ask) earlier this year.

His tail is starting to grow back,
but on the end of his tail (end of his tail), their is this thing that looks like a human blister, small,round and clear (basically see through).

let me explain the next part

do You know how the tail is split into two forks (top part and bottom part)?
On the bottom part of the fork (where his blister is), all the blood veins in that part of his tail, seem to been infuriate and lead to this boil.