black moor goldfish cannot eat

Question
My black moor fish cannot find his food even the food is just right in front of him. His mouth seem to be open all the time unlike the others. He used to be the one that eat a lot but not the past 3 days. What should i do?  

Answer
There are a couple of things that come to mind:

1. his water is not clean and clear enough and he is getting sick from too much nitrate etc and needs a water change.

2. He is in a small bowl or small tank and does not have enough room and is stressed

3.  He has some gravel stuck in his mouth making it hard to shut his mouth and blocking his ability to eat.

4. He's not hungry

I would suggest doing a 50% water change and making sure he has at least 10-15 gallons to himself.  If you have other goldfish, you need 15 gallons plus 10 gallons per extra fish.  If this doesnt' help him, try looking to see if he has gravel stuck in his mouth, if he does try and remove it