tank is crashing!!!

[hawaiiguy]

Thanks for all the advice everyone, I didn't think I'd get such a great response! So, I'll change about 30% of the water when I get home tonight and do a bunch of testing.
Dufferdan, I've been trying to catch my bicolour for a few days now... Sometimes I can flush her out of the rocks, but she still has enough energy for a fast break back in! And yes, she does have a patchy spot at the top of her head just in front of her dorsal fin. It's not a growth or anything, she's just lost her colour there. Its not her regular vibrant yellow, it's a dull whitish yellow colour.