Goldfish swimming top of tank, not eating?

Any advice appreciated - I have 3 goldfish (2 comets, 1 oranda), oranda has had slightly opaque fin ends, so I put in anti-finrot medicine. That seems to be clearing up but none of them are eating and they are spending a lot of time at the top of the tank, with their top fins a bit flattened. They're not gulping at the surface though. I usually change 1/3 of the water once a fortnight, I know many fish owners recommend 10-25% once a week, but they are all over 5 years old and that seems to work for them. I also 'hoover' the gravel when I do a water change. I was on holiday last week and I suspect the person who has been feeding them was giving them too much. Don't know if that has any relevance but thought I'd mention it anyway. Going to do a water change (last one was just over a week ago), maybe buy a water test kit. Any thoughts?