Change in behaviour in goldfish

Question
Hi, I wonder if you can help. I have 7 quite large goldfish in a garden pond. Whilst 6 are behaving quite normally, 1 has developed a strange pattern of behavior. It frequently has it's head turned downwards and appears to be eating food from the bottom of the pond although I can't tell if it is actually eating. Then it will swim very quickly to the surface moving in a way that looks as if it is trying to shake off something attached to it. It has been behaving like this for about 2 weeks.

Answer
Hello Karen,
         You are concerned about one of your fish, It looks like? I'm not sure you need to be alarmed. What I mean is, I have two Koi's & one Goldfish. All three of them do that all day long. Thats pretty normal behavior if you ask me. If you are really concerned about the Fish, isolate him. Take him out of the pond for a few days and put him in a Hospital tank. Make sure the tank is aerated correctly and that the tank is big enough for him. See how he does in the Hospital tank. If everything seems to be okay with him, put him back in the pond.
       Thank you for contacting me. I really appreciate it.
 Thank you,
           Shelly-