I have one zebra danio left over from having 7. I got them about 5 months ago and one by one they all died off except this one. Since then I've got white mollies and put them in with him. He has been fine for all this time with the other fish. The last cople days I've noticed that he has been inactive; swiming around in the same spot and not darting around the tank as usually. I'm was waiting for him to die off before but I've grown attached to him. What can I do to keep him alive?