I recently noticed one of my 5 white skirt glofish isn’t eating the flake food or blood worms. He’ll only eat shrimp brine, but I only feed that twice a week. He seems smaller than he used to be and slightly less active than others. All parameters are perfect. I did normal, every 2 week, 20% water change/half tank vacuum when I noticed this, last week, but no change. We got all of them about a year ago. Could he just be getting old?