Have you added any new fish lately? What is the temperature of your water? When you say the koi are all dead and the goldfish are fine, it makes me thing of a terrible possibility. Koi Herpes Virus kills only koi and leaves goldfish untouched. KHV is a herpes virus, so the virus can live in a koi for a while before it breaks and causes the disease. Think about how chicken pox works in humans. As a child, you get chicken pox. Then you recover, but the virus still lives inside on the nerve endings. Years later when stressed, the virus fires back up and causes Shingles, a painful disease of the nerves. And a person with shingles can give chickenpox to a child. KHV acts the same way. When a pond is infected with KHV, some of the fish die and some survive. The survivers are now carriers and will infect other koi at some point. All fish that survive KHV should be destroyed and the pond sterilized to prevent reinfection of this deadly disease. The bad news is that if it was indeed KHV, the goldfish are now carriers as well. Since you have no koi left to test though, there is no way to confirm that it really was KHV. Here's a link to a web site that explains more about KHV. Here is a link that might be useful: KHV web site

Oh darlin', how stressful for you and the fish. I thought about lots of woo-woo that could be going on but do you suppose they are hiding in plain sight when they see what they think might be a threat? Or perhaps Shark just still needs a rest once in awhile in a spot he thinks is safe and Spectacles is commiserating? Prazi is pretty potent stuff. Do the math again to eliminate all the stuff in the pond. It may be closer to accurate than you think. If nothing else it will keep you busy and not obsessing. I needed to use it only once and remember being a nervous wreck about getting it right but it worked like a charm. Now after all the warnings they used to put out, they say it is gentle and you don't have to repeat it. Guess it has been a long time since I used it for the pond. I'm not sure why you used it though. Do you know for sure they had parasites or was it prophylactic? If that was it then a lower dosage might have been called for anyhow. Since I'm not a Vet I'm probably just blowing smoke. There are so many newbies here. Maybe it is time for a discussion about Praziquantel. I know I needed a refresher. I attached a link signed by Dr. Johnson. Here is a link that might be useful: tips on using Praziquantel.

After a dealing with a Heron situation and a over-feeding situation that led to fish dying ( possible fin rot and stress from being poke at.)--The addition of a $5 bag of solar salt from Menards really did the trick for my fish. Since adding the salt, I haven't lost a fish. Is this something I should be doing each spring? Also Mike , I am in North Aurora-I watch this raggedy a$$ Heron, who look like he had a fish hook was in his beak-stalk my pond--he ducked under a table, squeezed between chairs,do everything like a cat. When we came outside he slowly moved from us--Either this guy is starving or he is a real psycho! The pond has a net and a plastic fence so Psycho left empty beaked. Have you seen this fellow?

At this point, that koi is so weakened that any parasite preparation may put him over the edge. Plus, you don't know what parasites he has (if any) and there is no one chemical that gets them all. For now, you need to work on getting the pH up and stabilized with baking soda. There is a baking soda calculator link at the bottom of the page. that link also has a tab at the top for a salinity calculator. Get that sick koi salted to 0.3% over a couple of days and get his water heated and aerated. Koi cannot heal when the water is cool...he really needs to be warm or he may not recover. PH shock is pretty hard on them. Salt the rest of the pond too, to help any remaining fish. Since the pH crash probably killed off your beneficial bio bacteria, you may see some ammonia spikes and a raise in nitrites. In addition to helping the fish overcome the affects of the crash, salt will prevent damage from the nitrites. Also, what state are you in? The akca.org has koi health advisors located all over the country and if you contact a koi club in your area they may have a KHA who lives close enough to you to come over and help. Here is a link that might be useful: Baking Soda Calculator