Very good, thank you!
+1 A common or Comet Goldfish, like what I assume you have, can easily get over a foot long. Easily. They can get 2 feet long. And need double filtration because they produce so much waste.
should have done your research! Now a poor Goldfish has to suffer because of it. Get rid of him, please. I bet being dinner would be better for him. That is pretty selfish to say that. You need to care for him.
Next time, do your research. Now you ither need to make things right or rehome him.