My yellow tann just stopped eating...No visible signs on it's body...no labored breathing...no erratic movements...just will not eat. It has been almost two weeks it has been behaving this way...just swims around the tank. I have been feeding Nori and a mix of frozen foods. All of the other fish eat like there will never be another feeding. I have had this fish for about 4 years and this is the first time I have seen this behavior. It has always been a active feeder...Any ideas?