Something is wrong with my archer fish?

I've had a four inch archer fish for about two months now, and for the last few days he has been acting different. He is getting really dark in coloration and the whites of his eyes are a brown color and he has become real skittish. I've tested the water and everything seems normal. He is still eating but not as aggressively as he used to. I have no idea what is wrong with him or what to do? Any suggestions would be great!