Goldfish constantly keeps mouth open

[shemmen14]

My goldfish swims around everywhere with his mouth wide open. It started happening about a day ago. He can still eats and swims fine, but sometimes he isolates himself from the other goldfish in the tank. I attached a photo of what he looks like and there seems to be some kind of substance in the middle of his mouth which may be related to the issue, but i really don't know. All help is appreciated.

 

[the loach]

He probably has a piece of gravel stuck in his mouth. You shouldn't have this type of gravel in with goldfish of this size.

 

[FreshyFresh]

Can he close his mouth at all?

Like loach said, a finer substrate is best for goldies as they like to sift through it by nature.

If it is something lodged, maybe you can try holding him in a wet paper towel and get it out with tweezers.