"Nitrite was next to Zero" isn't good. What's the ammonia level? I wouldn't worry about hardness at all, but it would be nice to have actual values rather than "very high". The ProForm-C isn't be used with salt. I don't know what your salt level is, only what you added/plan to add. Formalin, part of ProForm-C, lowers O2 in the water and salt increases slime on the gills making it more difficult for the fish to take in O2. At some point these two issues can kill stressed fish. It's hard to tell what salt level in your pond would start killing fish. Throwing stuff into the pond without knowing what problem your fish actually has...very good way to kill fish. When healthy they can take that kind of needless abuse, but when already stressed, it can be a real killer. It can be very difficult to figure out what the problem is, but for certain you get make a small problem into a huge problem. Step 1. Water quality. If ammonia and nitrite aren't zero that's what you fix. You do nothing else. Fish, like people, can often recover if in a good environment. Fish immune system is the best tool you have for figuring out a problem and fixing it. Step 2. Add an air pump. It's safe. It may not be needed, but if it does increase O2 it will enable the fish to better fight whatever the problem is. Step 3. If water is cold, say under 60F, you can move sick fish to warmer water in a hospital tank. Slowly warming the water. Fish immune systems don't work very well the colder the water. Optimal I think is 75F but you'd have to check that. Step 4. Try and figure out the actual problem. Step 5. Treat the actual problem in a hospital tank. If you can't figure out the problem or a hospital tank seems like too much hassle but you really want to dump something into the pond go ahead. Then go shopping for new fish. Here is a link that might be useful: Videos on helping sick fish

I went through a similar experience after purchasing goldfish from and on line vendor but they were being held in quarantine in a 400 gallon pond when they started displaying nearly the same symptoms and resulting death. I determined that my fish were suffering from some unknown parasite. I used Proform C which is a broad spectrum malachite green & formalin treatment which will eliminate nearly all parasites that afflict goldfish or Koi. Read the directions carefully and treat your whole pond three times doing water changes between treatments. Worked wonders for me and came very highly recommended at that time. I tried salt myself and they still kept dieing since salt is only effective for a few parasites why waste your time. Once treated as prescribed you will not need to drain the pond since all parasites should be eliminated. It is important to have a fairly accurate knowledge of the volume of your pond. One quart will be enough with plenty to spare. Let us know how it all is resolved. I was so happy that I quarantined these fish and not released them into my main pond of 8000 gallons with many other fish until I determined they were healthy and free of parasites.

Professionals say fancy goldfish tend to just do this sometimes. Thanks anyway.

He's dead now. Why do my new goldfish keep dying right after the 2 week warranty period? while a tankmate might keep going fine for months. Still, none live as long as goldfish are supposed to live. It looked like swim bladder, but he never ate enough while in my care to have overeaten himself into a blocked swim bladder. And he can't have been just a weak fish, because someone kept him alive and healthy for the couple years or however long it took to get him to the 2" size I bought him at. The only fish stores in this area are the big boxes. I won't buy fish at PetCo because all their fish die within two weeks, so I go across town to PetSmart, their fish live longer than two weeks as often as not. Not this one. He was perfectly healthy looking when I got him, I always watch the fish carefully before making a choice. And he was Gorgeous. Such a waste, to take that gorgeous creature home and kill him. So he was fine when I got him, but he never really ate for me, and wasted away and died. This has happened many times in the past. Is that just how it goes with fish these days? I don't remember having this problem when I had a tropical community tank back in the day of independent fish stores, pre - PetCo. I feed them fairy moss and mosquito larvae and milfoil from my little 25 gallon outdoor water garden. Could they be contaminated with something that's killing the fish? I never add anything except fish safe pond products to it, and not much of that, but it is Rubbermaid material, with plants in plastic nursery pots. the fairy moss came from a nursery but it lives in the pond. The milfoil came from Lake Washington but I washed it carefully before adding it to the pond, and then to the aquarium. What would milfoil carry that would be toxic to goldfish? The oranda is still fine, but now lonely. I've had him a few months now but I'm afraid to get him another companion until I know why my new fish keep dying.