That's great. While I was waiting, I calculated your inches of fish per gallon of water and that is fine, too. The water temperature is perfect. Since two fish ahve died with no symptoms other than lethargy and gasping for air, we have to assume there is something internal causing the problem. There ares several possibilities. There are some viral diseases of koi that cause death by destroying the internal organs. Fish often die with no signs of anything being seriously wrong. These diseases are more likely if you have added new fish in the last few months. There's a slight chance that birds or frogs can transfer it from one pond to another. If this is what's wrong, there is nothing you can do. In that case, more koi will come down with the virus, but the goldfish will not.
Another possibility is that the fish have a parasite not visible to the naked eye. Gill flukes are one that could cause these symptoms. The flukes are microscopic, so you can't see them. Adding pond salt is a good idea because it will help the fish with stress, but it won't actually treat any disease. Often, when gill flukes are present, a bacterial infection is also involved. The recommendation is to first treat with an antiparasitic. I like Praziquantel, which should be available in any pet store.Treat once according to the directions, then follow up with a second treatment 4 days later. After the second treatment, do a water change, then treat with an antibacterial. A combination of Melafix and Pimafix is good. Another good solution when we can't be sure of a diagnosis is to treat with TetraPond Pond Fish Treatment, along with the salt.. It treats a range of problems encountered in pond fish. If a virus is responsible, however, it will have no effect. Here is an online source. They do ship internationally, but there would be increased shipping costs.http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=5408
You've already taken a good step by increasing aeration. I would also do a partial water change of about 10%. Then add the salt. If the sick fish isn't better in 24 hours, go ahead and either treat the pond or move that fish to a hospital tank to treat with medications.
If you have more questions, let me know in a REPLY. I hope this will turn out to be something treatable and your fish will recover.
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