Hmmm, thanks for the suggestions guys. Wish there was a definitive answer, but either way he's still pretty. The two pics are actually only seconds apart, so the color change isn't stress. It's due to the lighting in the tank. One side was only lit by an actinic bulb because the other had just died.
I may have to get rid of him though as he quite quickly become the tank bully. Since he's grown in size I've found a few fish just gone missing randomly. I go to bed at night with seven yellow labs, come home from work the next day and there's only six. Then another couple days go by and there's only five left... And so on. And the only culprit I can see is this guy. He's the only one chasing fish around. And the odd part is he's not trying to breed, not defending a territory either. A fish will just swim past and it's like a switch is flicked and it's war! Lol. If I lose any more fish, I'll be putting up an ad to a good home!
