My anglefish looks very ill. I mean he is up, swimming about, eating etc. but the problem is he has lost all of his colour! he is transparent! He is a half black, and I'm worried now, because I have two Half blacks, and the other one is slowly turning colourless too. They are in a 400L tank, with four other angelfish who seem to be fine. These angels are fine in every way apart from the colour, what's going on? someone please help me.