Whats wrong with my Green Terror?

[wLpetty3]

75gal w/ fluval 404.
In the tank are 1 8"GT, 1 12"Oxydoras Niger, 1 Yoyo Loach, 1 small Pleco.

Usually the GT is a hog and will punk on the catfish when it comes out from behind the log. Then one day I cleaned my filter, and did a water change something Ive done many times before. Only difference this time is I added carbon as the water was yellow from the wood. All the other fish are cool except the GT. Now I dont know how the carbon could have done anything. But now the GT wont eat, it get pushed aside by the cat when I feed. (hikari cichlid gold and cichlid staple) It developed a swollen gut and cheeks and breaths heavy. and I guess whats sticking out of his anus is his "organ"
His color is unchanged and he swims upright like normal.

So I dont know whats wrong, just worried that he has not eaten for 2 weeks. I have not tested the water yet I've only added aquarium salt. He looks faded in the pics but his color really is bright. I read somewhere a fish can go into a sort of rut or heat when then want to spawn. Any truth in that?

here above you see its "organ" and a white string like discharge.

 

[Otherone]

Interesting - If I had to take a stab at it I'd say protozoa maybe capillara aka round worm. They are not the same as white stringy poop which is more idicative of a sucken belly - the rounds worms grow and gather to the point of bloating and the white discharge is thinner more clear like mucus. The fish eat like hogs because the nutrients are being robbed. I'd increase oxygen add salt , consult with the Fish disease forum and remove the carbon, start treating the tank with Piperazine or Prazziquantel. Round worms can take years to show their ugly selves.

However their always the possibility it is sexually originated, this I know nothing about. Sorry and best of luck.