We recently added a Scribble Angelfish into our 500 litre tank but he hasn’t been eating. He seems to just be swimming around one area if the tank. I did notice his mouth looking a bit odd but I’m not sure if it might have anything to do with him not eating. We feed our fish mainly Brine shrimp and marine mix, I just tried feeding him Spirulina and Mysis but he weirdly was shaking his head rapidly like he tries to eat but couldn’t. Could it be mouth rot? Cotton mouth? (Don’t mind the dirty tank just about to do a change)