First thing you need to do is get the sick fish out of the tank for at least two reasons... to possibly save it from the other fish killing it and to prevent it from getting the others sick. It's never good to leave a sick fish in its community. Then, you can try to figure out what it has and maybe medicate it accordingly. Good luck.

Thanks for your info! But what's bothering me is that I'm a new comer to fish keeping. I have no idea what is causing the problem. I will keep it in a separate place but don't think it will become health again. I need some proper treatment.

Now my fish is acting more weird - kept on rotating left and right like it can't find a balance. It keeps on going around and tries to settle into the cracks and gaps between the gravel. Once it did, which is usually in a upsode down position, it will gasp for water and filter air out of the water very rapidly. I need help emergently!