Hi everyone, Im new to the group

Ok, tank info, 50 gallons, side filter, 4 large (ish) goldfish. However, the
smaller of the four spends most of his time hiding under the filter, have no
idea why, because he seems healthy, and so do all the others, he does come
out sometimes, and the other fish seem ok with him, but I am wondering why
he hides?

Thanks
David