Weird goldfish behavior

Question
Greetings.

I have a rather large goldfish (3.5" long), and in the last week or so he's been acting rather odd. If you look over at the tank, he'll have his fins clamped close to his body, or  will be swimming rapidly to the bottom of the tank and bounce off the rocks, swim to the top of the tank, and do it repeatedly. I was concerned about this, but when you walk over to the tank, his fins pop back out to a normal position and he acts perfectly normal. He's eating fine, water parameters are all within normal limits, and nothing has changed.

Do you have any insight to this behavior?

Answer
Hello,

Usually the cause of clamped fins is stress. That stress can be brought on by a lot of things, such as inappropriate water quality/parameters or illness. Since you said the water parameters are normal, I suspect he has an illness such as ich. I would treat with a broad-spectrum bacteria/fungus medicine, and raise the temperature to eighty degrees Fahrenheit if possible. A little aquarium salt may help as well. Good luck!