Hippo tang help


My hippo tang in my 280g DT has been very un active lately. For over the past week I barely see him and when I do he is basically just hanging out in one spot. His colors still look great, he's plenty fat and still occasionally eating just been very inactive and reclusive lately which is not like him at all. The fish is around 7" long and has been in the tank for almost 2 yrs now. All parameters test good and I havn't had any changes to the tank recently. Every other fish and coral is thriving. Anybody have any ideas on what could be causing him to act this way???