Big Blister on Goldfish

Question
Hello, I have had the same goldfish for over 7 years. We have had several different fish in the same tank and many have disappeared, one jumped out of the tank and others have died. I noticed the goldfish, aka Wonka, has this big white blister like bump on his head. It's been there for a about 6 months, at least, and looks to me that it's gotten bigger and may have popped. Do you think this had to do with any of the other fish that were in the tank with him. Some were other goldfish, some were tropical fish. Those fish have been gone for over a year though. I don't know if it hurts him or not and I feel bad. What are your thoughts?
Thank you!

Answer
Hi Melissa,
 That is really hard to say without seeing the fish directly and even then, it might not be obvious. Fish can get "blisters" for a variety of reasons, ranging from mechanical damage such as banging into something sharp or being bitten by another fish.  Or it could be the result of a parasite or other disease.  

-- Ron C.
  rcoleman@cichlidresearch.com
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