what is the circulation like in your tank? this is not a bubble breeding nest it is excess protein film that is forming on the top of your water from not enough circulation, is there kind of an oily film on top of the water as well? i get this sometimes in my small tank when i have to divide it and the flow isn't as strong. and with that many fish in a 55g you should definitely be doing water changes at least weekly, are large are they? unless they are small grow outs they need to be moved to a larger tank asap