I have a similar problem. My black moor is lying on the bottom, and has been there all day (about 10am when i first spotted her, its 3pm now). I added a fungal remedy when i first saw it and have just started a 20%
water change.
The weird thing is she was lying inside the ceramic jug ornament this morning, and the other goldfish was lying next to her. Keeping her company, how sweet. And now that the moor is lying outside the jusg, the other one swims around and has eaten (the moor tried to but didnt eat) and occasionally goes back to the moor and sits beside her. Its so sad but cute.