I take it that this is fairly new behaviour? Has anything changed in the pond which happened about the same time as this. Apart from staying near the surface is the fish otherwise ok, you mentioned it is still feeding but is it still fully alert and react to stimulation i.e if you try to touch it would swim away in the normal way. Can the fish swim below the surface if it wants to or is it very buoyant and unable to do so. Finally have you tested the water if so what are the results of the tests. Out of curiosity how deep is the pond.
Buoyancy problems have two main causes, the most common is a digestive problem where gas becomes trapped in the fish's gut, it's quite common among fancy goldfish but less common with other fish although it does still occur. With 'normal' fish the problem normally clears with in a few hours. The fact that the fish is still feeding well also suggests that there is no blockage or discomfort there. And catching the fish to treat it in a separate container may cause to much stress for a large 13 year old fish but this could be tried as a last resort if the situation continues unchanged for a long time. Epsom salt added at the rate of one level teaspoon per four gall of water, this will act as a laxative and it may help.
The other cause is a swim bladder problem due to a malformation, infection or trauma all of which seem unlikely in your case. There actually isn't rally a great deal that can be done to help, there are no meds or potions which are likely to help so it is probably just a case of wait and see. In the mean time you can help by making sure that the fish does keep going below the surface so that its dorsal region doesn't dry to much and become sore, and if possible offer some shade because fish can become sun burnt believe it or not.
Incidentally it is always worth checking the water when anything untoward occurs just to rule out a few things.
I'm trying a couple of days without feeding the fish, apart from peas, to see if that makes any difference. In fact, I might pop in to my local fish centre at lunchtime to see if there might be a sinking food to use instead of floating sticks. I wondered if taking in a lot of air when feeding from the surface could be making the fish gassy?
She does get a little sun burnt and ends up with a whitening on her back and dorsal fin.
I will also get some salt to add to the water and do a water test.
Just wanted to update on here as still concerned about my mirror carp. Even now the weather has cooled down significantly and obviously so has the pond temperature, the mirror carp is always still on the surface with her back out of the water. She seems very "buoyant". She can swim underwater, but when still, always seems to be at the top. She still eats really well and looks visibly healthy. Any ideas anyone?
Unfortunately buoyancy problems can be many things. Can even be something silly like they ate a piece of styrofoam that was small enough to swallow, but too big to pass. Very unlikely, but just can be lots of things.
The fish doesn't bob to one side or the other? To me, and this is purely opinion not science, a fish with gas in the gut will be on its side sometimes, have trouble keeping upright, because the gut is in their underside. Also, because this has lasted so long I wouldn't be thinking gas, virus, bacteria...doesn't mean it couldn't still be these things...just guessing.
Feeding peas was a good idea. It's safe and can work for some causes. Always worth trying.
Loved that finnipper suggested not using salt and you took that advice. In so many pond forums salt gets thrown at everything.
Sinking vs floating food wouldn't matter. Both foods would be equal. It's the breakdown that causes the gas, if any. Or blockages. However, you could give the fish more vegetation. Only a very small chance it could help, but at the very least the fish would probably enjoy it.
I don't have any cures to suggest. Feeding peas is about all we have short of taking the fish to a Vet for xrays. But even with an autopsy these buoyancy problems often remain a mystery. I've cut open more than a few fish that had problems like these for a long period and always think I'm going to find something obvious but haven't yet. I usually just say "it's genetic" which is my way of giving up and just let the fish live out its life.
The sun burn is the main unfortunate problem at this point. Adding shade is the only help I can suggest for that.