You should test the ponds water quality for ammonia, nitrites, nitrates and ph with and accurate test kit/s and post the results back here ASAP. Fish will produce excess slime when their protective slime coat is irritated by irritants in the water, so its vital that you test the ponds water (even if it looks clean, stuff like ammonia and nitrites are not visible to the naked eye, so you need to test the water).
I would advise refraining from feeding the fish food until you get the water quality tested, as goldfish produce a lot of waste and if you feed them lots of food when the water quality of the pond could be potentially bad, you will only make this worse. What is the temperature of the pond as well? Feeding goldfish when the pond temperature is too cold can give them serious digestive problems.
Is there anyway you can put one of the sickest looking fish into a plastic transparent fish transport bag so you can view the fish more closely? Its important to know for sure that the fish isn't skinny, bloated, has inflamed/sore looking gills or anus, clamped or tattered looking fins, stuff coming out its bum, lice or stringy things stuck to the fishes body etc. If it has any of these symptoms i would definitely suspect a parasite or disease is making the fish ill, although its very important to accurately ID what is wrong with the fish, as treating the fish with unnecessary medicines could only make it worse.
Is this pond filtered at all and how many goldfish do you have in the pond? How long, wide and deep is the pond as well and what is its general shape (round, square, oval etc)? Its possible that since your goldfish spawned last year, there are now too many fish in the pond for the water quality to remain well, and so the less hardy goldfish are falling ill- IMO 90% of all goldfish illnesses stem from undesirable water quality or stressful conditions that come about in their habitat, even if these things are not immediately obvious to us etc. More info on your goldfishes situation would be very useful in determining exactly whats wrong with your fish
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