Need some advice

I need a little advice about my goldfish. I have two “feeder” goldfish, which are about three inches now. They have been doing great for the most part but every few months they exhibit very strange behavior. They won’t eat and dart around the tank frantically even when I turn the light on. They will also hide which they haven’t done since I bought them a year ago. Then after a day or two they are fine and go back to normal. They are in a 20 gallon and share it with 2 snails. Tank parameters are normal. If anyone has any advice or an explanation I would appreciate it. It always makes me feel bad when they do this, I don’t want them to be stressed. Thank you.