Need Help Understanding Bacterial Infections

[baboon]

I have a Purple Tang and two Clownfish. My question is how can you tell if your fish has a bacterial infection. I don't understand the symptoms. My Purple Tang hides a lot and twitches often a long with some other strange antics. He will go to the top right corner of the tank about two or three times a day, for very short time periods. All my water parameters are fine with my nitrates at about 20ppm. One of my clownfish also acts wierd occasionally which leads me to believe they might have a bacterial infection. The thing that confuses me is that they all eat very well and "appear" to be in good health. I was hoping that someone could help me in understanding the signs and symptoms that these fish in particular might display if they had an infection, especially the Purple Tang. Also what medications might I use if they do have an infection.
Lastly what about being contagious? Are bacterial infections able to spread to other fish in the tank if one fish is infected. If they are contagious should I leave my tank empty for a certain time period as you have to when a tank has ich?
I have absolutely no clue as to what bacterials consist of, so all information would be greatly appreciated. Thanks.

 

[baboon]

Upon further observation it seems as though my Purple Tang might be throwing up (for lack of a better word) his food. As I thought about it more I have not seen him "normally" digest his food in as long as I can remember. What could this possibly be from. I was hoping this might help if nobody could answer before.

 

[baboon]

bump