If you added fish a couple of weeks ago and they weren't quarantined beforehand that's soon enough for them to have introduced parasites or a bacterial infection without you knowing it. The latter has an incubation period, so no symptoms may be presented at first.
And parasites often take awhile to multiply to numbers significant enough to show visible gill irritation. All it takes is a couple of gill flukes (which are not necessarily noticable), on one new fish to introduce them to the others. And once the parasite has access to several fish they'll multiply quickly.
At this point I'm going to suggest you treat with an antiparasitic (prazipro is one that will treat various parasites). It will not hurt your tank (though I've heard of them harming shrimp, so you may want to move them temporarily) and will have a beneficial effect if your fish do have parasites. You put it directly in the tank.
If you cannot find Prazipro try to find something like geltek antiparasite medicine which can be added to the food (dumping this med directly in the tank does not work well, it must be consumed).
Those are the 2 I use and find to work best. If for some reason you can't find either look for ANY parasite/gill fluke med. Anything will be better than allowing the infestation to continue.
I would treat with an antibacterial as a last resort, and those are best used in quarantine tanks because they can kill your biological bacteria.
The symptoms you describe could definitely be attributed to a heavy gill fluke infestation. Often you will see them on the skin and gills (they look like little white nodules, or tiny white worms) ,but not always.
