Check your water parameters. These guys are really sensitive. When I had my 4 in QT, I forgot to pre-treat the water before a water change. I realized in the midst of adding the water and ran to get the Prime from the Kitchen. By the time I got back, one was dead and the others were exhibiting the nose out of the water, as you describe.
Something might be burning their gills, so check for ammonia and nitrite if you haven't already.
By the way, my original three are now going strong, healthy and happy. I got them around Thanksgiving. They didn't really color up until I moved them from QT into my heavily planted display tank. I really think they need a lot of plants to be healthy.