I would say stress. Put him by himself for a while or with some cheap dithers (or punching bags) and let him chase them around or beat them up a bit and make him feel better and dominant again.
When my 2 juve flowerhorns would go at it the loser always turned black and hid in weird positions up against the glass. I think they show their best colors when they are dominant. So try that mirror thing too, that will make him feel dominant again.
When my 2 juve flowerhorns would go at it the loser always turned black and hid in weird positions up against the glass. I think they show their best colors when they are dominant. So try that mirror thing too, that will make him feel dominant again.
