So I've got five Long-Fin Zebra Danios, and today while looking at them, I noticed one has picked up a very golden hue across his (I think) whole body. Not an illness looking hue, just like the silvery-white in his body is now yellowish-gold. It also seems to be accompanied by a little more agression as he is VERY vigorously chasing around his tankmates. Normally it is one of the females (I think) that does the chasing. Is this a normal thing or should I be concerned about his health? Possibly a preparation for breeding?