Senior Panda acting weird

[Exemptednato]

So I originally had two Pandas in one tank for about 2.5 months. They lived side by side and happy. One of them died and now the other just sits next to a branch and moves only when food flakes drop near him. He dosent seem scared at all, but mabye depressed?

After a week, alone, to see if he would move, he continued his same routine so I got two smaller pandas and added them to the tank. Those two school together, but when they try to play or school around the branch, the Senior Panda just seems very uninterested, so those two smaller pandas just ignore him now. It's been 2.5 weeks since the death and he's still doing this roving branch thing. Can a panda Cory be depressed, and for that long? 2.5 WEEKS?!

 

[Exemptednato]

Update: I'm positive the tank conditions are not related to his behavior, the Temp is optimal, with a nice filter, and the Senior Panda has been living in the same tank since I bought him as a baby. I do the same routine to clean it and he's never had an issue. His issue arose when his buddy Panda died. That day and forward he just sits alone along that branch. I checked for sickness, and he definitely isn't sick. He still eats and swims around the branch, just has no interest in schooling ever since his buddy passed