I had an angelfish die about 6 weeks ago, which was paired with another angel. Since it's death, the other angel has been "sulking"... It hides beneath plants, it runs into the tank walls if I walk by the tank, and whenever I try to feed the tank it swims off and hides or slams into the sides of the tank trying to get away. Is there anything I can do to help this fish? Should I isolate it for awhile? It is the largest fish in the tank and no one ever messes with her. She's over 4 years old as well.