Well, there's a couple of things that could be going on.
1 ) your new fish could have brought some kind of disease into the tank and shared it with all his new tankmates (how kind of him - heh!)
2 ) your old fish could be stressed out about having new tankmakes, therefore lowering their immune systems and they are then sick...
My best guess is it's a combination of both.
Therefore, it seems like they probably have some kind of fun parasitic infection. One such type of infection is called chilodinella - the description of it is: an external parasite causing the fish to secrete excess mucous, resulting in the appearance of a bluish-white film on the body of the fish. In severe conditions, the skin peels off, gills are damaged and fish dies.
Does it seem like your fish are gasping for air?
Are any of your other fishes scratching on rocks?
Although your fish doesn't exhibit all of those signs, in addition to scratching on things, it sounds like there's a mucous buildup going on.
Parasites also cause fish to scratch on things.
SOOO.... if I were you, and had the things going on that you do, I would treat for a parasitic infection. Maracide is very effective for stuff like that.
I would like to have others hop on this topic and see if they concur with me - I'm not an expert in these types of fish, I only have bettas, but the meds and research i've done pretty much apply to most fish in most cases.
Good luck, and let's hold off on the treatment until someone else pops in and gives their 2 cents worth.
