I personally think your overfeeding. Once they get lazy about food you know they are either eating too much or something is up. Since I feel like your on the ball with maintenance I am going to say they are well fed and are not hungry. Go a few days without food and see what happens. Or switch up the diet a bit more. add variety never hurts.
Without seeing your tank and fish its super hard to say for sure what is up but my first thought would be chill out with the feeding and let their bellies flatten out for a few days. Keep an eye on their poops too, look for anything weird.
Without seeing your tank and fish its super hard to say for sure what is up but my first thought would be chill out with the feeding and let their bellies flatten out for a few days. Keep an eye on their poops too, look for anything weird.