Jax,
Two things come to mind when you described thier behavior. One is that tyou said "fat" goldfish. Are they fantails, lionhead, oranda, or some other thick bodied goldfish? If so, I have found that these varieties are not nearly as cold hardy as comets, shubunkins, and some other varieties. The other thing that comes to mind is something I have seen in frozen ponds here in mid winter before. Many people here have rock or even gravel bottom lined ponds that are often filled with lots of pots, plants ect. These ponds tend to collect organic matter (fish poop, leaves, and other waste material) that fills nooks and crannies in the rocks, gravel,
and plants. To make matters worse the circulation is low or nearly non-existant in winter. These materials continue to rot even in winter and without the O2 circulating through these areas they go anarobic. This can raise the hydrogen sulfide gases which affects thier gills and makes them gasp at the open water holes in the ice. I have seen die-offs of all the fish in ponds that are over 3 years old that have not been stripped to remove the sludge. A good telltale sign is the rotten egg smell you get when moving rocks and plants around in the pond. Having koi, I have found that I have far fewer problems with fish health and water quality in winter and early spring by cleaning the ponds thourghly in fall instead of spring like most people do. Hope this helps.